The point? Even godless people may manifest fine qualities. But if you choose them as your close friends, your thinking, faith, and conduct will be affected. Thus, in his second letter to the Corinthians, Paul stated: “Do not become unevenly yoked with unbelievers.”— 2 Corinthians 6:14-18.
lacking a common faith does reduce the positive relationship that is only available to those who share a common faith, belief and practice.
Paul made a very valid and important point.
(2 Corinthians 6:14-18) 14 Do not become unevenly yoked with unbelievers. For what fellowship do righteousness and lawlessness have? Or what sharing does light have with darkness? 15 Further, what harmony is there between Christ and Be´li·al? Or what portion does a faithful person have with an unbeliever? 16 And what agreement does God’s temple have with idols? For we are a temple of a living God; just as God said: “I shall reside among them and walk among [them], and I shall be their God, and they will be my people.” 17 “‘Therefore get out from among them, and separate yourselves,’ says Jehovah, ‘and quit touching the unclean thing’”; “‘and I will take YOU in.’” 18 “‘And I shall be a father to YOU, and YOU will be sons and daughters to me,’ says Jehovah the Almighty.”
While the "darkness, Belial, idols", do not apply; the thought is true that such a lack of closeness in the critical matter of belief, will reduce the quality of a marriage.