This metaphor is taken from horses or oxen which, being joined together by a common yoke, must walk and pull together in the same direction and with the same goal or have serious problems. The reason believers and unbelievers should NOT MARRY one another, form BUSINESS PARTNERSHIPS together, seek to have close SOCIAL FELLOWSHIP or CONDUCT RELIGIOUS SERVICES and projects together is that they do not have the same principles, natures, nor goals. They cannot walk together in harmony because they are not going in the same direction and they are not agreed on the vital issues of life, sin, salvation, God's glory, and the gospel of Christ. If a partnership is formed, a marriage entered into, or a close fellowship established, serious problems will very soon appear; for "what fellowship can righteousness have with unrighteousness? What communion can light have with darkness? What harmony can there be between Christ and the devil?" The Apostle Paul, in these verses, is not just setting forth rules and laws concerning communion with unbelievers. Paul is wondering WHY ANY BELIEVER WOULD SEEK SUCH ALLIANCES, and what possible agreement or fellowship could come of these relationships! Believers and unbelievers have very little, if anything, in common, certainly nothing that would promote peace and the glory of God.
"Blessed is the man that walketh not in the counsel of the ungodly, nor standeth in the way of sinners, nor sitteth in the seat of the scornful; but his delight is in the law of the Lord, and in His law doth he meditate day and night" (Psalm 1:1-2).