Union with God

"But whoever is united with the Lord is one with him in spirit", 1 Corinthians 6:17. This expression has a similarity with the following texts: "That all of them may be one, Father, just as you are in me and I am in you" John 17:21 (spiritual gift of concord); "This is eternal life: that they may know you, the only true god", John 17:3 (contemplation).
The concept of union with God is related with the contemplative life (John 17:3) and the concord (John 17:21). The eternal life is a spiritual gift, the eternal life consist in a set of blessings that God give in advance to us, one of this blessing is the contemplation. The contemplation is a special meditation, a direct relationship with the divine, contemplation is devotion.
The Concord instead is a special type of friendship, is the union of wills that arises from the charity, the love for the good.
Be one in spirit with the Lord is share with him the good love, the love for the good, the love of charity.
http://www.quintoevangelio.com.ar/en/articles/item/99-union-with-god.html