1 Corinthians 7: 39
The last sentence in that verse says marry only in the lord.
They is more to that verse than we can think of, the scripture says marry only in the lord it have a good reason.
To illustrate
A farmer who plough the ground always use beast of the same size and height, if is a donkey, it will be two donkeys, if is a bull, two bull. But if a farmer uses a donkey and a bull to plough his farm it will be a disaster one will drag the other it won't be a balance combination.
Explanation
If a Christian want to marry a Christian he/she should go for someone who he/she shear the same believe with. Imagine a Muslim marrying a Christian the equation is not balance they will end up being a misunderstanding along the way base on what they believe, is only on a rare occasion that the combination works.
A woman today tell us her story and regrets for marrying someone who doesn't shear the same believe as her's. I felt her pain in words.
Bible is older and more experienced than any human. So is not joking when it says: marry only in the lord.