Because the Holy Spirit is God, and therefore our union with Christ is inseparable, it means God will never regret saving us.
This part is more than a theological point of our security. It has to do with how we grasp it. It’s not enough to say that we are inseparably united to Christ. But how does God feel about that? What good would inseparable union do for us right now if we thought that God was unhappy about it? What tempered good is eternal security if we thought God felt stuck with us?
Because the Spirit unites us to Christ, it means that all of Christ’s benefits become our own. His sin-defeating death was where the guilt and power of our sin were defeated (Romans 6:3). His death-defeating resurrection was where the power and sting of our graves were overcome (Romans 6:4). His vindication as God’s Son is a vindication in which we
In Christ, as the Spirit himself bears witness, we are God’s children (Romans 8:16–17). We are fellow-heirs with Christ
— beloved of the Father like he is. And this kind of love from this kind of Father is not begrudging.
Because we are united to Christ, and the bond of that union is God the Holy Spirit, then even in our worst moments, in our lowest lows, in our deepest darkness, God has never regretted saving us. Never.
He chose us for the praise of his glorious grace (Ephesians 1:4–6); he made us his own workmanship (Ephesians 2:10); he will complete what he began (Philippians 1:6) — all of which the Spirit guarantees (2 Corinthians 1:21–22). It really is true: God will never regret saving us. He is irrevocably glad in the inseparable union we have with Christ that he has accomplished by his Spirit.
Want to read the entire post? You can find it at http://www.desiringgod.org/blog/posts/god-will-never-regret-saving-you .