It is found in 2 Corinthians 6:14 that we are not to be unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?

Let’s take a minute and break this verse down with a brief look at the definitions (Webster) so we can gain a clearer understanding of what this really means. ‘The phrase “unequally yoked together” is the translation of just one Greek word, heterozugeo, which is a compound word that means, “to yoke up differently; to associate discordantly; unequally yoke together.” It is used but this one time in the Bible.’ (bible-truths.com)

-unequally: not like or not the same. Badly balanced or matched
-yoke: to become joined or linked
-unbelievers: a person who does not believe something; especially : a person who does not believe in a particular religious faith
-fellowship:common; the relationship of people who share interests or feelings

So, looking at the definitions we can determine that we are not to be linked to people or have intimate relatioinships with those who do not believe in God.

In my opinion, I believe being unequally yoked does not only apply to being bonded to atheist or nonchristians, but also those that don’t have a passion pr a mind for Christ. Here is an article that breaks it down a little further.