Be you not unequally yoked together with unbelievers: for what fellowship has righteousness with unrighteousness? and what partnership has light with darkness?
All Commentaries on 2 Corinthians 6:14 Go To 2 Corinthians 6
Basil the Great
AD 379
These words clearly indicate an act which is absolutely forbidden and is displeasing to God and perilous for the one who would venture to commit it.