There you have it: Don't marry an unbeliever—that is, someone who doesn't share the basics of Christian doctrine and practice.
But yes, avoiding being "unequally yoked" is an excellent biblical principle. Paul advised the Christians at Corinth to avoid entering significant relationships, such as marriage, with unbelievers.
I want to go back and re-quote what I said during the Feast so you can understand the background of all this.
What comes to mind when you hear the term unequally yoked? 14 Do not be unequally bound together with unbelievers [do not make mismatched alliances with them, inconsistent with your faith].
Elisha Marie Levy provides some insights into what the Bible says about the matter.
Richard Ritenbaugh warns that dating outside the church is fraught with obstacles and potential dangers, yoking a believer with an unbeliever and exponentially complicating the spiritual overcoming and growth process, exposing one to perdition or providing a grievous cross to bear.
It is impossible to have the best of both worlds (the world and God's way).