Paul goes on to remind believers that they are the dwelling place of the Holy Spirit, who inhabits their hearts at salvation (2 Corinthians -17).
Because of that, they are to be separate from the world—in the world, but not of the world—and nowhere is that more important than in life’s most intimate relationship—marriage.
I think it is true that our young people date too young, and that many dating situations are unwise in that they create unnecessary temptations and, even if nothing wrong takes place, a couple’s actions may give the appearance of evil.
The principle stated in 2 Corinthians -18 would seem to apply to marriage as well, and some might see it as applying to dating.
The passage goes on to say that there is no harmony between Christ and Belial (Satan).
There can be no spiritual harmony in a marriage between a Christian and a non-Christian.
I suspect that this would be an inferential conclusion, and thus would have to be considered a matter of personal conviction, rather than as a clear biblical prohibition.
One could certainly ask, “If a Christian cannot marry an unbeliever, why would they ever date one? I think one can have a friendship relationship with an unbeliever of the opposite sex, without entering into a dating relationship.