Scripturally a Jew is anyone who has renounced idolatry and thrown in his lot with the people of the one true God. Historically there have been three rites involved in receiving proselytes into Judaism: circumcision, water immersion, and a sacrifice.
The central ritual of admittance into Judaism has always been a baptism by water immersion. The sacrifice was never as important as circumcision or baptism, especially after the Temple was destroyed, making sacrifice impossible. Furthermore, since women converts to Judaism far outnumbered men, circumcision could hardly become the chief rite of entry into Judaism.
The one indispensable thing that any convert had to do to become a Jew was to get baptized.
Proselytes crossed the threshold into Israel through an immersion, because Israel had entered the Promised Land through the water of the Red Sea. In the same way, all enter God’s Promised Land through water baptism.
There was a definite concept of cleansing or purification built into this decisive immersion. A heathen who left behind the idolatry of the Gentile world to become a Jew had passed from sin to a whole new life. When he came up out of the water, he was considered ritually clean, beginning life all over again with a clean bill of goods, like a child newly born.
The baptism of proselytes or converts washed the uncleanness from the heathen on entering Judaism. So non-Jews were grafted into the people of God by a water immersion, which gave them ceremonial purity.