There is ONE people of God through all of history.
This one COVENANT PEOPLE had the sign of the covenant given them, albeit in a different form.
The sign or figure changed clearly. OT circumcision was the sign of the covenant given to adult males, proselytes and their infant male children. However it goes without saying that the true Israelite (Rom.2:28-29) is one circumcised in heart and his elect seed. Today it is also true that not all who are baptised as professing adults or the infant children of one or two believers is regenerate. But the fact remains Abraham and his physical seed and the infants of that physical seed were commanded to have the sign given them. The OT washings were also signs of God’s covenant with the people, young and old (Deut.29:9-17, Ex.24:8). Point is, adults, children AND infants were baptised and this fits exactly with the NT norm of FAMILY BAPTISM.
The Old Testament baptisms or washings were types of the New Testament’s one baptism. There is harmony between the testaments and an expectedness and acceptance of John the Baptist’s and Christ’s disciples’ baptisms. There were NOT two modes dipping in OR sprinkling but rather one mode-dipping in and sprinkling!