Even before Christ's Passion, Baptism, inasmuch as it foreshadowed it, derived its efficacy therefrom; but not in the same way as the sacraments of the Old Law. For these were mere figures: whereas Baptism derived the power of justifying from Christ Himself, to Whose power the Passion itself owed its saving virtue. [emphasis added]
If Baptism justifies us, it is not a power that it holds intrinsically, but rather that it holds from Christ; hence, apart from God working through it, it would have no power whatsoever to save. A discussion of how God uses the sacraments to impart grace to us is not exactly relevant in this series. The point for our purposes here is that the mere act of Baptism does not justify us; rather, it is God working in the sacrament by his grace that justifies.
An objection might be raised to the fact that God works our justification by means of this human act. This is misplaced. Already we have seen that faith and repentance are necessary for salvation. The Protestant must necessarily believe this himself if he accepts the testimony of St. Peter's Pentecost sermon. Human participation is not contrary to grace in these respects; rather, it is the fruit of grace.