A good friend, and fellow church member, wrote that God was the Father of all mankind. However, if that were true, then how does one explain John 1:12, which says, "But as many as received Him, to them gave He power to become the sons of God, even to them that believe on His name." If all human beings were children of God, then the verse makes absolutely no sense.
While there are many passages in the Old Testament which declare God to be the Father of Israel (Ex. 4:22; Mal. 1:6; etc.), it is clear from His discussion with His Jewish opposition, that not all Jews were children of God; He declared, "Ye are of your father the devil, and the lusts of your father ye will do..." (Jn. 8:44).
The Apostle Paul's epistle to the Romans states that a person is not a child of God because he is from the lineage of Abraham (Rom. 9:7); if that were true, then all those born through the line of Ishmael would also be children of God. He continues, "That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed. For this is the word of promise, At this time will I come, and Sara shall have a son" (Rom. 9:8-9). In other words, those who believe that God would provide a Savior through Isaac, would be God's children.
"And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish My covenant with him for an everlasting covenant, and with his seed after him" (Gen. 17:19). Salvation, being a child of God, is based upon faith in the promise of God to produce a Savior through the lineage of Isaac, and as we all know, Jesus is the fulfillment of that promise (Mt. 1:2; Lk. 3:34)!
Believe in the Seed of Promise; become the Seed of God!