It is certainly true that when children of believers reach the age of discernment [and have never repented or believed] they will have alienated themselves from God and destroyed utterly the truth of baptism. But this is not to say that our Lord has not elected them and separated them from others in order to grant them His salvation. Otherwise, it would be in vain for Saint Paul to say that a child of a believing father or mother is sanctified, who would be impure if he were born of and descended from unbelievers (1 Cor. 7:14).
John Calvin, Treatises Against the Anabaptists and Against the Libertines pg. 52