What are the Unequally Yoked?

Be you not Unequally Yoked together with unbelievers: 

for what fellowship hath righteousness with unrighteousness? 

And what communion hath light with darkness?  

And what concord hath Christ Jesus with Belial? or 

What part hath he that believe with an unbeliever? 

And what agreement hath the temple of Almighty YHVH (YEHOVAH) with idols? 

for you are the temple of the living Almighty YHVH (YEHOVAH); as Almighty YHVH (YEHOVAH) has said,

"I will dwell in them, and walk in them; 

and I will be their ALMIGHTY, 

and they shall be my people".

Therefore go out from among them, and separate (yourselves from they), says the Almighty YHVH (YEHOVAH), and don't touch the unclean thing; and I will receive you, and I will be to you Father, and you will be my sons and daughters, says Almighty YHVH (YEHOVAH).

2 Corinthians 6:14-18