The prior existence of faith as deserved merit, could never be proved — not even before Baptism. That is why the Belgic Confession (art. 21) explains: "We do not mean that faith itself justifies us."
However, faith is indeed the implied prerequisite for all advocates of Adult Baptism alone. Indeed, the [Adult] Heathen — who have as yet had no participation in the Covenant — do need to profess their faith before receiving Baptism.
Professor Lee brings out in bold relief that even children of believers "while yet in the womb -according to Calvin — have been adopted into the Covenant of eternal life." Compare too also the Canons of Dordt, I:17. What a comfort for so many worried and wondering parents!