God becoming human was the start of His being at-one with humanity. Then, on the day of Pentecost, Christ poured out His own Spirit on the disciples. They could then be one with their ascended Messiah in ways that they never even dreamed. The Spirit of Christ has been given to us to make us at-one with God. The Spirit is thus essential to atonement.
“The glory that you have given me I have given to them, that they may be one even as we are one, I in them and you in me, that they may become perfectly one, so that the world may know that you sent me and loved them even as you loved me.” John 17:22-23 (ESV)
