Note 13 at 1 Cor. 6:17: In a similar way that sexual relationships produce a physical union, our relationship with the Lord creates a spiritual union. This union is not just in purpose. Our spirit becomes one with God's.

The Greek word that was translated "one" here is "heis" and it means "a single (one), to the exclusion of others" (W.E. Vine's). It is the same word that was used twice in 1 Timothy 2:5 to say "For [there is] one God, and one mediator between God and men, the man Christ Jesus." We don't have multiple Gods. There is just one God. Likewise, when we get born again (see note 2 at Jn. 3:3, p. 92), our new spirit becomes one with God's Spirit.