The account of Acts 15 shows that it is not necessary for salvation for Christians to be physically circumcised. Salvation is by grace through faith, and faith without works is dead. We cannot "work" or "earn" our way into the Kingdom of God. The privilege to be a part of the eternal Kingdom is one that God bestows according to His will. Nothing we can do will make God indebted to us or require Him to do something for us, such as grant us admission into His Kingdom.
But at the same time, if we are within the salvation process, we must show forth works, or fruits, that demonstrate our repentance, our attitude, and our desire to live by the rules of His Kingdom. We must live now in the same way that we will be living for eternity—by the laws of God's Kingdom. Our works do not save us; they demonstrate that we are being saved.
Under the Old Covenant, the ceremonies and ordinances were primarily physical, and the spiritual aspects were implied. Under the New Covenant, the ceremonies and ordinances are primarily spiritual, and the physical aspects are implied. For example, there is no record of Christ ever performing an animal sacrifice, even though the Old Testament requires one in the morning and in the evening. Under the New Covenant, the physical rite is not required, yet the basic law is still there, and is thus manifested in morning and evening prayer, a sacrifice of our time and energy.
In the same regard, the council of Acts 15 shows that circumcision is not one of the works that is required to demonstrate the salvation/sanctification process. When considering eternity and the spiritual bodies that we will have at that time, circumcision is almost insignificant. What is truly important is whether or not the heart has been circumcised. The physical rite was a reminder to the children of Israel that they were separate and distinct, but even in this God was looking for a change of heart so much more so than a modification of the flesh.