Union with Christ is the irrevocable work of the Spirit. Once united, nothing can separate us from Christ. Nothing can make us a little more or a little less united. Union with Christ is unalterable. Communion with Christ, on the other hand, can be affected by sin and unresponsiveness to God’s grace. It’s like marriage: you can’t be more or less married (union) but you can have a stronger or weaker marriage (communion).
