Assuming no adultery, if an unbeliever divorces a believer because of his/her faith, is the believer free to remarry?
Yes. According to 1 Cor. 7:15-16, if an unbelieving spouse wishes to end the relationship, the believer should not resist and is not bound in such circumstances.
But if the unbeliever leaves, let him do so. A believing man or woman is not bound in such circumstances; God has called us to live in peace. How do you know, wife, whether you will save your husband? Or, how do you know, husband, whether you will save your wife?
However, the believer does not have a similar right to end the relationship. As long as the unbelieving spouse is willing to stay, the believer is bound. (1 Cor. 7:12-13)