In his covenant with Abraham, God promised that he would make Abraham the father of a multitude of nations, where God would be their God. Interestingly, the sign of this highly-universal-in-outlook covenant is the circumcision, which was particularly applicable for male only, thereby perhaps hinting at the current state of Abrahamic covenant (at that time), i.e., it was not fulfilled yet. Circumcision, in some way, testified that the Abrahamic covenant was still looking for its fulfillment.
For Christians, we believe that Abrahamic covenant has been fulfilled in Jesus the Messiah. The people of God is now characterised by our participation in the death and resurrection of Jesus, which is signified by our baptism — which indeed is applicable for all, not only for male anymore. “For as many of you as were baptized into Christ have put on Christ. There is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus.” (Gal 3.27-28)
Therefore, baptism is a testimony that Abrahamic covenant has been fulfilled. God has created a worldwide family with equal standing through the death and resurrection of his Son.