Modalism is a collective name for a group of doctrines which had their heyday in the late second and into the third century, but which were ultimately rejected by the Christian church. Modalism asserts that the One God revealed himself in salvation history as three forms or “modes” of operation. This idea arose, like many heresies, from good intentions. Its major proponents were Noetus, Praxeas and Sabellius and their concern was to defend the unity of the Godhead against tritheism. Tritheism is the idea of three separate Gods and is definitely not what orthodox Trinitrianism teaches, despite ill-informed statements to the contrary. Nevertheless, taking the sense in which God is Father, Son and Holy Spirit to the extremes of individuality could imperil a correct understanding of God’s fundamental unity. As in many situations where there is a tension within a doctrine (human free will versus God’s sovereignty, for example) extremes can beget extremes and one part may stray into error out of a misguided or untempered concern to avoid the opposite error.
As the early church began to really wrestle with how the divinity of the Son and the Holy Spirit could be understood in relation to the Father, some theologians were worried about the church adopting tritheism. Concerned to uphold the unity of the Godhead, they promoted what was termed monarchianism (a mon-arch is a sole ruler). Nevertheless, they did not want to deny the clear teaching of Scripture that the Son and Holy Spirit are divine. So the monarchians taught that the one and only God revealed himself in creation in different ways at different times; different “modes” of divine revelation. There were different forms of modalism, but they all expressed different terms for the same God, with no difference except his appearance and activity.
According to modalism, when God is revealed as Creator and Lawgiver, he is referred to as “the Father.” When he is revealed as Saviour in the person of Jesus Christ, he is “the Son.” As Sanctifier and giver of eternal life, he is termed “the Spirit.” Some forms of modalism were chronological and some were functional. Chronological modalism, also known as Sabellianism, proposed that God was Father at one stage of history, then he became Son and finally Spirit. In contrast, functional modalism held that God presently acts in different modes and that Father, Son and Spirit are merely different modes of action. Modalism in its different forms then, recognises one essence or being of God, but does not sufficiently discriminate between Father, Son and Holy Spirit, as the orthodox doctrine of the Trinity does, in keeping with Scripture. In fact modalism led to the idea that it was the Father himself who suffered on the cross (patripassianism).
The Bible clearly illustrates the distinctiveness of the Father, Son and Holy Spirit, whilst presenting each as divine. The Father sent the Son (John 8:42; Rom 8:3) to redeem fallen creation and Jesus spoke of God as his Father and submitted to him (John 5:30; Phil 2:5–8). He acknowledged that his authority came from the Father (John 7:16–17). At Jesus’ baptism, the Holy Spirit descended “as a dove” upon Jesus as the Father proclaimed his Sonship (Matt 3:16–17). Ironically, the Christadelphian position as originally defined blurs the distinction between Father Son and Spirit, approaching their own version of modalism, with claims such as “[Jesus] was the flesh-embodiment of the Eternal Father by the Spirit;” “The Father (by the Spirit) veiled himself in the flesh, and the result was Jesus of Nazareth…” “Christ being the Father veiled in our flesh is styled the Deity;” “The Deity and His Spirit are one… The spirit is but the infinite extension, so to speak, of Himself…” “Jesus was the true God in manifestation” 
This brings us to the Christadelphian doctrine of “God manifestation,” which they believe encapsulates not only the bestowed divinity of Christ, but the entire purpose of God with his creation. Regardless of whether many Christadelphians today could explain or defend the doctrine of God manifestation, or whether it remains their prime soteriological focus, it is still their official doctrinal position. Christadelphians believe that the deity manifested in Christ is that of the Father, not the Son. They understand that the attribution of deity to Christ is God the Father’s manifestation in the person of Jesus Christ, who although foreordained in the mind and purpose of God, began his existence at his conception by the Holy Spirit and his deity was his by bestowal from the Father, not intrinsic to his being as the one who shared his Father’s glory prior to being sent (contra John 17:5).
The expression “God manifestation” comes from 1 Tim 3:16 in the King James Version (KJV), which reads, “And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory.” The word “manifest” appears extensively in the KJV and in nearly all cases it is a translation of the Greek word phaneroo, to manifest or reveal. More modern versions tend to translate phaneroo as “reveal” or “make known.” Two related words are epiphanaio (to appear) and epiphaneia (the appearance, particularly of a deity) which Paul uses to refer to the appearing of Christ, especially in the pastoral epistles. Another key verse is John 17:6 (KJV), where Jesus says to the Father, “I have manifested thy name unto the men which thou gavest me out of the world.”
The doctrine of God manifestation was very much the cornerstone of understanding of the person and work of Christ by the “pioneers” of the Christadelphian tradition, such as John Thomas and Robert Roberts and detailed in C.C. Walker’s Theophany: A Study in God-Manifestation. Renewed interest in the doctrine emerges periodically. 
God-manifestation is the display of the glory of God. It is God’s fundamental purpose with His creation (Num 14:21), and is associated particularly with the revelation of His character and attributes . . . When Adam fell because of his sin, he no longer displayed the image of his Creator as he had previously done; and so he became estranged from God (Gen 1:27; 3:24). God’s ultimate purpose is that this estrangement of mankind from Him should be reversed, and that, in bringing about human salvation, He should make ‘a new creation’ of men and women in whom His glory will be finally and fully displayed in all its moral and physical perfection.” 
Christadelphians believe God has revealed himself since the fall by means of intermediaries, “God manifest in others,” and through his divine name, Yahweh. Yahweh is best translated, not as “I AM,” but as “I will be Who I will be.”  According to Chrstadelphians, Who God will be is the multitude of the redeemed, immortalized people whom he is preparing as manifestations of his name, of which Christ is the forerunner, the firstfruits. Jesus is regarded by Christadelphians as the ultimate manifestation of God, in that he is a man in whom the fullness of God dwelt, but he is still a human intermediary, the culmination of a long succession of men and angels who “manifested” God’s glory by speaking God’s words and displaying aspects of his character (glory). As the Word made flesh, Jesus was the thought, mind and purpose (logos) of God expressed in speech and personally in real human flesh. He had no authority or intrinsic divinity, no light of his own but manifested the character and words of God the Father.
By service and suffering he was made perfect, a full manifestation of his Father (Phil 2:7–8; Heb 2:10; 5:8–9). He has therefore been raised to the manifestation of God in Divine nature, inheriting a name greater than angels, and glorifying God in the process (Heb 5:5; 1:4; Phil 2:9–11). Jesus is now the anointed Son of the Father in a greater sense (Acts 13:33; Heb 1:9). He carried the name “THE LORD (Yahweh) OUR RIGHTEOUSNESS” (Jer 23:5–6) and, particularly when he returns, men will bow to the Father through him, and his glory will be revealed (Isa 45:23).”
The Christadelphian position is that Christ is divine only in a derived sense and that his exaltation is a reward for his obedience, not a return to a former status. Salvation for human beings then involves their manifestation as sons of God, of whom Jesus Christ is the firstborn. This requires people to receive the Word of God and believe and obey a body of truth. Then with baptism they become children of God. By abiding in the doctrine of Christ, believers manifest the Father and the Son, being sons of God now whilst being conformed to the image of his Son. The future manifestation of these sons of God will allow them to partake of incorruptible divine nature (at the resurrection) and reign in glory with Christ upon the earth. 
This idea of God being manifest in a multitude of saints, who are made co-heirs with Christ who is the full manifestation of the Father, might seem a reasonable and interesting perspective at first. However, there are two major problems with this doctrine. Firstly, Jesus Christ is merely the ultimate human manifestation of God in a line of men and angels; he is not intrinsically divine. Secondly, apprehension of Christ’s saving work is by imitation, effectively by works, for which believers are rewarded.
According to Christadelphian C.C. Walker, the Name Yahweh is borne by the angels; they are the elohim, or “mighty ones,” and they speak as if they are God himself. This, he asserts, is equivalent to the way Jesus speaks with direct authority, because he came in the Father’s name, spoke God’s words, but was not actually God, any more than the angels are.  Unfortunately, this comparison ignores the fact that no angel has ever sat on God’s throne or received worship there, which is the prerogative of the Lord Jesus, so the comparison is actually invalid as an explanation of Christ’s status. The idea that the angel of the Lord can be closely identified or equated with YHWH is not exclusive to Christadelphian expositors, by the way. There are various theories among Old Testament scholars regarding the identity of the angel (messenger) of the LORD, the malak Yahweh. Whether the term to use is “God manifestation”, or “God representation” or “God’s messenger” or some other expression, what we cannot assume that these passages teach is that this means that the way Christ manifests Yahweh is exactly the same as the way the angel manifests Yahweh. After all, Christ is not an angel; he far surpasses them.
Walker expresses the Christadelphian belief that Jesus is merely the first of many “sons of God,” setting the pattern of “probation before exaltation,” in a very works-based exposition of the Christian’s relationship to the saving work of Christ. Christ is an example to follow in the struggle for glory, rather than the One whose sacrifice is all-sufficient. For Christadelphians, Christ died as a representative of the human race, not as a sin-bearing substitute. The “manifestation of the sons of God” is seen as the culmination of God manifestation, which began with God manifesting himself in his representative messengers, and most fully in his Son Jesus Christ. Those who are in Christ will become a “multitudinous Christ,” God manifested in elohim, mighty ones, through the resurrection, of which Christ is the first fruits. The KJV verse critical to this interpretation is Romans 8:19: “For the earnest expectation of the creature waiteth for the manifestation of the sons of God,” however “manifestation” in this verse is actually not phanerosis, but apokalypsis, unveiling. The whole passage is about the sharing of Christ’s glory by adoption through the indwelling Spirit of him who raised Christ from the dead, not God “manifestation.”
Nevertheless, it is appropriate to emphasise God’s self-revelation throughout salvation history and his glory being manifested through his redeemed creation, with his most complete revelation in the person of his Son, Jesus Christ (Hebrews 1:1–2). There is a point in history at which God began to dwell among us (Gal 4:4), nevertheless Christ’s Sonship transcends his conception to include his pre-existent involvement in creation and his eternal relationship with the Father and Holy Spirit. Such is not explained by Christ being merely a man in whom the Father was manifest. The Christadelphian doctrine actually blurs the distinction between Father and Son by making Christ the manifestation of the Father, not the incarnation of the Son, and trespasses into the arena of modalism.
God manifestation is an insufficient explanation for the attribution of divinity to Christ, not only following his exaltation but during his ministry on earth. It also fails to explain the teaching of Christ’s humbling himself, being sent by the Father, and rejoining the Father in the glory they previously shared (Phil 2:5–11; 2 Cor 8:9). Viewing Christ’s lordship of all and his worthiness to be worshiped as gifts bestowed on him does not do justice to the principle that only God is worthy of worship, and will not share his glory with another, no matter how exalted one of his creations might be. God-manifestation, as ingenious a unifying theme that it may be, does not “prove” that the Son’s divinity is derived, rather the Christadelphian portrayal presupposes this. It makes Jesus out to be merely a prototype super-human or a super-angel. But the Son of God is superior to the angels (Heb 1; Rev 22:8–9). As a central, all-encompassing meta-narrative of salvation history, God manifestation falls short because it downplays the centrality of Christ’s work on the cross, the hopelessly sinful state of humanity and the all-sufficiency of the atonement (Eph 1 & 2; Col 1:13–20). Ultimately, in the face of Jesus Christ we see all the fullness of the Godhead in a sinless and perfect human being (Col 2:9), the one who is, literally, God with us, and who is Lord of all, worthy of all praise and worship and who has reached down to his wayward creatures with love and undeserved favour.
References and notes:
1. Alfred Nicholls, citing various Christadelphian pioneers, in Remember the Days of Old (Birmingham: The Christadelphian, 1978), 40–44.
2. Christadelphian Statement of the Faith, article I. http://www.christadelphia.org/basf.htm
3. For example, Stephen Green, “God Manifestation,” in The Testimony Handbook of Bible Principles, ed. Reg Carr, (King’s Lynn, UK: Testimony, 2010), 53–56 and in works available through Logos Publications, South Australia. Interstingly, Harry Tennant doesn’t mention it at all in The Christadelphians, What they Believe and Preach (Birmingham: The Christadelphian, 1998)
4. Green, “God Manifestation, 53
5. Although John Thomas is credited with this insight, many scholars have independently concluded that the Name is in the future tense. However, this of itself does not support any particular interpretation of the significance of that Name. For example, Raymond Abba argues that ehyeh has the sense of happening or becoming, or being present, and is used In Exodus 3 as an assurance of the presence of the Saviour God with his covenant people (Abba, R. “The Divine Name Yahweh.” Journal of Biblical Literature 80 (1961) 320–28). David Freedman discerns the original meaning of ehyeh as “I bring into being,” and the form in which it appears in Exodus 3:14 as “I create what I create.” (Freedman, D.N. “The Name of the God of Moses.” Journal of Biblical Literature 79 (1960) 151–56). Peter Enns argues that “I am who I am/I will be who I will be,” ehyeh is a prelude to the name “I am,” , “I will be with you.” (Enns, P. Exodus: The NIV Application Commentary. (Grand Rapids: Zondervan, 2000) 102–3).
6. Green, “God Manifestation,” 59
7. Green, “God Manifestation,” 55–56
8. C.C. Walker, Theophany: A Study in God Manifestation, (1929. Repr., Birmingham: The Christadelphian, 1967), 27
9. Walker, Theophany, 192–3