Just as God's revelation to all His people across the ages was revealed in progressive stages, by means of numerous covenants, even God's one covenant with Abraham was revealed in stages across the book of Genesis.
In Genesis 17, God reveals that the sign of His covenant with Abraham will be circumcision. But what does circumcision truly signify, and what does it not signify?
Circumcision is an outward physical sign, but it does not guarantee faithfulness. It was always meant as a shadow of the inward reality of regeneration, i.e. circumcision of the heart.
The tension this created was a mixed covenant community: a covenant community that, by design, contained some members who were faithful and some who were unfaithful. Unfortunately, there are many who have taken this tension and applied it directly to the New Covenant community, a.k.a. the church, insisting that the New Covenant community is also supposed to be mixed. And thus, they baptize their infants, just as the descendants of Abraham circumcised their infants.
But this is not what the Scriptures teach. The New Covenant Community is not defined by the physical sign, but instead by the inward reality.