The Incarnation of the Son of God is the chief foundation and demonstration of the union with God

"Abide in me, and I in you. As the branch cannot bear fruit by itself, unless it abides in the vine, neither can you, unless you abide in me. I am the vine; you are the branches. Whoever abides in me and I in him, he it is that bears much fruit, for apart from me you can do nothing." John 15:4–5 (ESV)