Note 13 at 1Co 6:17: Similar to the 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 spirits become 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" (Vine's Expository Dictionary). It is the same word that was used twice in 1Ti 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 are born again (see note 2 at Joh 3:3), our new spirits become one with God's Spirit.