It is utterly foolish for a believer to knowingly and willfully marry an unbeliever. The Word of God clearly forbids it, and the disobedient Christian cannot expect God's blessing on his or her marriage. 3 Paul is drawing upon the Old Testament law which prohibited Jews from yoking together two different kinds of animals such as an ox and a donkey. Because the animals were incompatible, the mixed yoke was unfair to both of them. For this same reason we are to avoid intimate unions with unbelievers.
Fellowship means sharing or participation.
This word refers to mutual association, close relationship, or intimate interaction. A partner who belongs to Christ and one who does not cannot make sweet music together. This refers to a share or portion which contracting parties hold in agreement. The potential for full agreement and complete harmony simply does not exist between believers and unbelievers, and it is therefore unfair to each party to yoke them together. If you are contemplating marriage to an unbeliever, pause and think with me.