6:14 yoked together. Paul is using a metaphor here based on Deuteronomy 22:10 and Leviticus 19:19, which forbade plowing with an ox and ass yoked together, or attempting to interbreed animals of different kinds. The clear inference is that believers and unbelievers are so different in character and interests (as well as ultimate destiny) that they should never be “yoked” together in situations requiring strong agreement of attitudes and goals (e.g., marriage, churches, business partnerships, lodges or other organizations with religious overtones). The prohibition is not intended to require complete repudiation of all secular or charitable organizations (note I Corinthians 5:10) or friendships. When one’s Christian faith is in jeopardy, or his Christian conduct and influence is endangered, then such connections should be severed. One can witness to unbelievers without partaking of their beliefs or sinful behavior.
6:15 concord. The Greek word for “concord” (sumphonesis) is the word from which we get “symphony.”