The unbeliever does not have access to God. He/she can talk about God and pray and study the Bible and act lovingly and selflessly, but if the Bible is True and God’s Word, then we know from Him that a non-believer does not have communion with God.
The believer has access to God.
How can two people, one who can commune with God and one who cannot, build a life together?
The non-believer does not have access to the Holy Spirit. It is through the Holy Spirit that God communicates, moves, expresses, and leads us. This does not mean that believers always act righteously or even appropriately. We do not. We are fallen. However, a basic and enormous difference between the believer and non-believer is the reality of the Holy Spirit.
The non-believer cannot worship the True God because his sin blocks his access. Onlky forgiven believers can worship the real God because we have been forgiven of our sins.
The non-believer cannot truly understand the things of God because sin is blocking his access.
The non-believer cannot lead the children from that marriage to the Lord because he himself is not a believer.
The non-believer is breaking the first of the Ten Commandments re: having no other gods before you. If God is not first in your life, you are in sin.
The non-believer married to a believer have no true common ground when it comes to life’s choices and vissicitudes. Without having the true God who is there to access and communicate with and struggle with, how does a couple weather life’s storms?