This series of verses is not an appeal for us to break all of our worldly associations. Recall that Paul urges the Christian partner in a divided marriage to strive to maintain the relationship as long as possible. This, instead, is an appeal to avoid too close associations. He says not to go into the world, but come out of it (see Revelation 18:4). We should not deliberately make close associations with the peoples of the world. It is all right to do business with them and to work with them, but avoid becoming harnessed together with them.
The statement, "I will be a Father to you, and you shall be My sons and daughters," seems to hinge on whether or not we are allowing ourselves to become yoked into these associations. God does not want us in these close associations with the world because it almost inevitably leads to compromise with His standards. It jeopardizes the consistency of the Christian's witness for God because there is a spiritual force in the world that undermines the Christian because the unbeliever does not share the Christian's standards, sympathies, or goals in life.
Is it unfair that God should ask this of us? Remember, He has bought us with a price (I Corinthians 6:20). The price was the life of His Son, which obligates us to a life of purity and holiness. Once we accept that sacrifice for the forgiveness of sin, we belong to Him. He is our Master, and He says, "Come out of the world and be separate." That is a demand that He puts on us.
Does God ever ask us for something that is not for our good? Of course not! And how is this for our good? Because He knows that it is likely that His people, though they have the Spirit of God, will have an extremely difficult time resisting the spiritual force that wants to lead them to compromise on the standards of His Kingdom. He thus obligates us to purity of life, to holiness, to separation from evil. We owe our allegiance to Him alone, and we cannot allow ourselves not to be a fit vessel for Him to live in.
There is no surer way to go backward in our spirituality, to blunt our feelings about sin, to dull our spiritual discernment until we can scarcely tell evil from good, and to dry up the source of our spiritual strength than by needless mingling with the world. We should stress the word "needless" because Paul writes in I Corinthians 5:9-10 that to avoid all contact with the immoral, one would have to go out of the world. There is nothing in the New Testament to indicate separating oneself by moving into a commune of believers or living alone like a hermit.