In mathematics, if A is a subset of B, and B is a subset of A, then A = B. Jesus is in the Father and the Father in him (John 14.11), so the Son and the Father are one. Union with God is, to be more precise, being one with God. Now God is a space infinitely larger than mine, how can we fit within each other? Physically, that would be impossible; but spiritually, it's no problem. When I surrender to God, I become entirely his and I am in him. At the same time, he enters my heart and we become one. Receiving the Holy Eucharist seals the deal, you could say. By the way, the Holy Eucharist had been the greatest aid in bringing me to this state. True, God is everywhere around us, but to bring him into your heart you have to open the door, to invite him in. There is no other way of getting more intimate with God in this life!