Page "Infant baptism" Paragraph 45

Covenant theologians point out that the external sign of the covenant in the Old Testament was circumcision.

Circumcision was performed upon the male children of Israelites to signify their external membership in God's people, not as a guarantee of true faith ; the Old Testament records many Israelites who turned from God and were punished, showing that their hearts were not truly set on serving God.

So while all male Israelites had the sign of the covenant performed on them in a once off ceremony soon after birth, such a signifier was external only and not a true indicator of whether or not they would later exhibit true faith in Yahweh.