For a Lutheran, Holy Baptism applied to an infant perfectly depicts the central article of the justification of a sinner by God's grace (undeserved kindness) through faith (trust in the promise of salvation in Christ) for Christ's sake.

As for "infant faith," Luther refused to speculate on things he could not know. Who can really know what an infant trusts? Baptism gives the person, whether infant or adult, something tangible to trust. It's nice to know that Christ died to save the world. It's even better to know in Baptism that He died "for you." The words "for you" require all hearts to believe.

I'll even dare to speak personally. (Yes, Lutherans have personal testimonials too!). I was baptized when I was five weeks old, was nurtured in the Liturgy, catechized by my parents and pastor, etc. As a result, I do not have a single conscious moment in my life when I did not know and trust Jesus Christ as my Savior. That's why I can't relate to "Amazing Grace" when it sings "I once was lost but now I'm found." I have to believe from the Scriptures that I once was lost, but I have no such memory or experience. I've always been found in Jesus.