What is the reason? And does God command it?
2 Corinthians 6:
14
Do not be mismatched with unbelievers; for what do righteousness and lawlessness share together, or what does light have in common with darkness? 15 Or what harmony does Christ have with Belial, or what does a believer share with an unbeliever? 16 Or what agreement does the temple of God have with idols? For we are the temple of the living God; just as God said,
“I will dwell among them and walk among them;
And I will be their God, and they shall be My people.
17 Therefore, come out from their midst and be separate,” says the Lord.
“And do not touch what is unclean;
And I will welcome you.
18 And I will be a father to you,
And you shall be sons and daughters to Me,”
Says the Lord Almighty.
NIV
14 Do not be yoked together with unbelievers. For what do righteousness and wickedness have in common? Or what fellowship can light have with darkness? 15 What harmony is there between Christ and Belial? Or what does a believer have in common with an unbeliever?
The believer wants to be in church each week. He/she wants to serve God , to glorify God in what they do for entertainment, music, what they teach the children regarding truth vs error. Etc. That is going to get in the way of what the unbeliever wants to do.