Who should believers not be unequally yoked with?

Answer: Unbelievers

Do not be unequally yoked with unbelievers. For what partnership can righteousness have with wickedness?

“Do not be unequally yoked with unbelievers. For what partnership can righteousness have with wickedness? Or what fellowship does light have with darkness?”— 2 Corinthians 6:14 (BSB)