I would also add, Hisway, that the physical things of the Old Testament, as real as they were, were also symbols of the spiritual truths, which are spiritual. The Ark of Noah was a 'type' of Christ. Joseph (one of the twelve brothers) was also a 'type' of Christ, as was Moses. Egypt was a 'type' for sin, and so it goes. All were real, but all were also used as pictures of a higher reality.
Circumcision was that same thing. The Israelites, being formed and chosen by God, were given a physical sign of identification with that. But God's Kingdom is NOT of this world, and so neither is the sign by which we, as Christians, are reckoned. Baptism in water is a fleeting thing, quite unlike circumcision which is a forever thing. Anyone could undress an Israelite and see the sign of who he was. That most certainly is not true about baptism in water. You dry off and that 'witness' is finished!
Our true and lasting mark is from the Lord Himself. It is our changed hearts. They know us by our love. And the saddest thing I know of is that so many claim to be Christians and show, rather, a spirit of hate.
The mark we have, from the Spirit, is our changed lives. The fact that we are now able to love God with everything in us and able to love our neighbor as ourselves -- these exist only for the born again believer, and it is this which is the permanent mark of our sonship in Christ.
Getting wet and drying off are not nearly enough.