So what did the Holy Spirit intend by His command not to be bound together with unbelievers? Bound together translates a participial form of the verb heterozugeo, which means, “to be unequally yoked.” Paul drew his analogy from Deuteronomy 22:10, where the Mosaic Law commanded the Israelites, “You shall not plow with an ox and a donkey together.” Those two animals do not have the same nature, gait, or strength. Therefore it would be impossible for such a mismatched pair to plow together effectively. Nothing in the context would lead to the idea that he is referring to earthy issues of human endeavors. In Paul’s analogy, believers and unbelievers are two different breeds and cannot work together in the spiritual realm. He called for separation in matters of the work of God, since such cooperation for spiritual benefit is impossible. The false teachers were eager to blend the people of God with the pagan worshipers, because that hinders the gospel. That is what this text forbids.
John MacArthur