“The modern debate over infant baptism frequently shows how far removed we are from the debates of the first century. Our debates center around a question like this: ‘Do you mean to say that you think the Gentiles in the first century baptized their infants? Where do you get that?’ In the first century the question was more like this: ‘Do you mean to say that the Gentiles don’t have to circumcise their infants?’ It was a foregone conclusion in the first century that something must be done with the infants — after all, if at least one parent was a believer, the children were holy (1 Cor. 7:14)” (To a Thousand Generations, p. 79).
You ask just the right question: when did the infant right to the covenant sign get revoked? It’s a question that the credobaptist cannot answer.