Question 22 of the Westminster Shorter Catechism asks: How did Christ, the Son of God, become man?
The answer given is: Christ, the Son of God, became man by taking to himself a body and a soul like ours, being conceived by the power of the Holy Spirit in the womb of the virgin Mary, and born of her, yet without sin.
The first observation that I need to make is that there is mystery here. We must acknowledge that complete understanding of this is beyond our reach as humans.
Secondly, note the process that is mentioned. Christ became man. The eternal second person of the Trinity took a body and became man.
There is at least one group, that claims to be Christian, that has this teaching the other way round. They teach that man can become god through following the teaching of their church. This claim can be found no-where in Scripture.
Jesus was conceived by the power of the Holy Spirit in the womb of the virgin Mary. Jesus was not a human child born of two human parents that somehow became the Messiah or upon whom was conferred deity. This is a heresy that was dealt with and rejected in the early church.
With regard to Jesus being sinless, I am reminded of one of the stand-out verses in the writing of the Apostle Paul. He states:
He made Him who knew no sin to be sin on our behalf, so that we might become the righteousness of God in Him.2 Corinthians 5:21 (NASB)
Jesus, who was born and lived without sin, took the punishment for our sin so that we might be viewed by God as having Christ’s righteousness.
Without Christ’s perfection, the idea of Christ being our substitute would fall flat and we would be left without hope.