“Don’t marry an unbeliever” is standard wise counseling for young Christians.  Second Corinthians 6.14 – “Do not be unequally yoked with unbelievers” – is the common biblical reference used.  Verses 14-16 give a list of examples of unequal yokes:

• Righteousness and lawlessness
• Light and darkness
• Christ and Belial (devil)
• Believer and unbeliever
• Temple of God and idols

Verses 16-18 cite Leviticus and Isaiah to make the case for being set apart:

• I will be their God and they shall be my people
• Go out from their (unbelievers’) midst and be separate from them
• Touch no unclean thing and I will welcome you
• I will be a father to you, and you shall be sons and daughters to me.

God is a jealous God and wants his people devoted to Him and His ways; history tells us that unbelievers will drag believers away from Him.