The Cow - 2:221
Do not marry women who associate partners unless they believe. A believing servant woman is better than a woman who associates partners, even if you like her. And do not marry your daughters to men who associate partners until they believe. A believing servant boy is better than one who associates partners, even if you like him. They call to the Fire. Allah, on the other hand, calls to Paradise and forgiveness, and He makes His verses clear to the people so that they may take admonition.
Why does Allah forbid marriage with those who associate partners?
The Women/4:116
Allah does not forgive associating partners with Him (if it continues until death), but He forgives the sins below that to whom He wills. Whoever associates partners with Allah has gone astray.
The Women/4:48
Allah does not forgive associating partners with Him, but other than that He forgives to whom He wills. Whoever associates partners with Allah has committed a great insult.
Luqman/31:13
"When Lokman advised his son, he said: "My dear son, do not associate partners with Allah; surely shirk (associating partners with God) is a great wrong."
The Table Spread/5:72
"Those who say: "Allah is the Messiah, son of Mary,"have certainly disbelieved. But the Messiah said, "Children of Israel, serve Allah, my Lord and your Lord. Whoever associates partners with Allah, Allah forbids him Paradise, and his abode will be the Fire. And there will be no helper for the wrongdoers."
The Light/24:26
Evil women are for evil men and evil men for evil women. Good women are for good men and good men for good women. These (the second group) are far from their rumors. They deserve forgiveness and a generous reward.
The Woman Who Is Examined/60:10
O you who believe, when believing immigrant women take refuge with you, question them. Allah knows their faith very well. If you realize that they are believers, do not send them back to the disbelievers. They are not lawful for those disbelievers, nor are they lawful for them. Give back to them their marriages which the disbelievers have spent. There is no harm in your marrying them if you have paid them their mahr. Do not keep unbelieving women in your charge. You may ask them for the rasham you have spent on them, and they may ask for the rasham they have given you. This is the judgment of Allah. He judges between you. Allah is All-Knowing, All-Wise.