Genesis 9:12 says, And God said, This is the token of the covenant which I make between me and you and every living creature that is with you, for perpetual generations:  After God made His promise to Noah to never again destroy the earth by water, God said He was going to provide a token, or sign, to remind people of His promise.  The sign that we have today to remind us of salvation through Christ is not just the cross, but the Holy Spirit living in us.  Anyone can make a cross, and some may claim to have splinters from the original cross, but only those who accept Christ as Savior and Lord have the Holy Spirit indwelling them.  Verse thirteen says, I do set my bow in the cloud, and it shall be for a token of a covenant between me and the earth.  God put a rainbow in the sky to remind people of His promise.  I had a friend once who said he didn't believe the rainbow was a sign from God, because people could make a rainbow.  I know that on a very limited scale, when conditions are right, we can produce a small rainbow, but God is still in charge of the conditions and the laws behind the rainbow.  Verse fourteen adds, And it shall come to pass, when I bring a cloud over the earth, that the bow shall be seen in the cloud:  It is not Mother Nature that is in charge of the weather, but it is Father God.  I don't believe that God dictates every weather event, but I believe that He can if He so desires.  Verse fifteen proclaims, And I will remember my covenant, which is between me and you and every living creature of all flesh; and the waters shall no more become a flood to destroy all flesh.  God did not promise that there wouldn't be floods that killed people, but only that there would never be another flood that destroyed all flesh.  Likewise, when we accept Christ as our Savior and Lord, God does not promise that we will never have problems, but promises that these problems will never destroy our everlasting life with Him.  Verse sixteen states, And the bow shall be in the cloud; and I will look upon it, that I may remember the everlasting covenant between God and every living creature of all flesh that is upon the earth.  I don't believe that God needs a reminder of His promise, but that this is simply a sign for people to know that God remembers His promise.  We can be certain that God will always remember and keep His promises.  Verse seventeen declares, And God said unto Noah, This is the token of the covenant, which I have established between me and all flesh that is upon the earth.  God told Noah this was a token, or sign, of His covenant.   As followers of Christ, as already stated, the sign that we have of the truth of God's promise to us is the Holy Spirit living in us.  We should need no other signs.  Verse eighteen says,  And the sons of Noah, that went forth of the ark, were Shem, and Ham, and Japheth: and Ham is the father of Canaan.  Then verse nineteen concludes,These are the three sons of Noah: and of them was the whole earth overspread.  These were the people, along with their wives, who would repopulate the earth.  As discussed earlier, those who see the mark of Cain as some physical trait that passed from generation to generation would have to acknowledge that none of Cain's descendants survived the flood, so even had there been a mark on his descendants, it would no longer be in existence.  Of course, I didn't believe that it passed to anyone else to begin with.