The Word of God is absolutely conclusive that for a Christian to marry an unbeliever is directly contrary to it.
We would not, however, dare to limit (as we have heard some do) 2 Cor. 6 to marriage. On the contrary, the passage strikes at every unequal yoke where Christian and unbeliever are joined together for any common cause.
There is no question as to the bitter persecution both from believers (alas) and unbelievers, a faithful adherence to such a course involves. Men will tolerate you as long as you treat all as equal, but to make distinctions between holy and unholy is offensive to them.