Do Christadelphians believe that Jesus is completely God and has been God since before the creation of the world?
If not, when does Jesus become God (or take on God like attributes)? What is their justification for this?
Quickly from Wikipedia:
Christadelphians hold a view of Biblical Unitarianism. (source)
But that leaves us with the question was is "Biblical Unitarianism"?
Today, biblical Unitarianism (or "Biblical Unitarianism" or "biblical unitarianism") identifies the Christian belief that the Bible teaches God is a singular person—the Father—and that Jesus is a distinct being, his son. A few denominations use this term to describe themselves, clarifying the distinction between them and those churches which, from the late 19th century, evolved into modern British Unitarianism and, primarily in the United States, Unitarian Universalism.
The history of Unitarianism was as a "scripturally oriented movement" which denied the Trinity and held various understandings of Jesus. Over time, however—specifically, in the mid-19th century—Unitarianism moved away from a belief in the necessity of the Bible as the source of religious truth. The nomenclature "biblical" in "biblical Unitarianism" is to identify the group/s as not having made such a move. (source, emphasis mine)
Although, I do not know anyone from this group, the definition of the group and their belief on the Trinity would seem to suggest that they do not hold the doctrine of the trinity.
I hope I can help here as I am a Christadelphian. It is correct we don't believe in the doctrine of the Trinity and our view is consistent with Biblical Unitarianism. This is an important point of distinction as other non Trinitarian denominations may hold a view of 'oneness' or binitarianism as their theology.
In direct answer to your question, no, we do not believe Jesus is completely God or in fact God at all. What we do hold is that Jesus is the Messiah, he is the son of God born of a virgin birth and was sinless and therefore had very much a God-like nature. We also believe that Jesus' teachings were the word of God. We stop short at saying he was divine. Jesus was a sinless man. I think that makes him more special. We know God can't sin. If Jesus was God, the fact he didn't sin would render insignificant.
In answer to the next part of the question. As a consequence of the aforementioned material, we do not believe Jesus was God or at least present in any way before creation. Jesus came into being when he was born to Mary. Jehovah's Witnesses believe Jesus was the angel Michael in heaven born on earth through the virgin birth. We do not hold this view at all. We are very much in line with orthodox understanding here.
Next part. Jesus never becomes God. He is God's son and has a God-like nature but will never become God. So when did he adopt a God-like nature? Great question. Many others have greater explanations than I. The simple answer is that we can't be sure exactly. The Bible says little of his childhood. We do know that when he went to Jerusalem for the Passover with his family, he stayed back at the temple. He was very learned in scripture for his age. Some say at his Baptism where God professed "this is my son, in him I am well pleased." There is however no doubt that in his ministry, God was pleased in all Jesus did (God resurrected him so he must have been happy).
My sincerest hope I have helped you. There are some brilliant Christadelphian pamphlets on the Trinity. You can order one from the website of 'The Christadelphian' (magazine) or view free copies online or pop into your local Christadelphian Church. Most have these pamphlets and would gladly hand you one for nothing more than a smile.
Read the Jesus, God the Son or Son of God pamphlet. This is the common position in Christadelphia and backs up what I've said above.
No matter if one believes in the Trinity or not, we all believe in the Father, Son and Holy Spirit. It is our doctrinal view which differs.