Rod Rosenbladt's lectures on Luther's Commentary on Galatians he pointed out that Lutheran and Roman Catholics have different definitions of faith. The Roman Catholic church believes that having faith means to cognitively arrive at the position that you believe what the Roman church teaches. The Catholic Encyclopedia defines faith by quoting Thomas Aquinas. It says faith is:
"the act of the intellect assenting to a Divine truth owing to the movement of the will, which is itself moved by the grace of God"Lutherans define faith as trust in God which is worked in us by God through His Word and sacraments. In previous posts such as this one, I've noted that Baptists think it is absurd that a baby could ever have faith. In MacArthur's recent rant, he talked about how crazy it was that Luther could believe that infants could have faith without ever really explaining why it is crazy to think an infant could have faith. I think the reason he thinks infant faith is crazy is because he's working with a definition of faith that is very similar to the Roman Catholic definition of faith.
Biblically, faith has nothing to do with cognitive ability. John the Baptist lept in faith in his mother's womb. The Psalmist speaks of hoping in God while on his mother's breasts and this Psalm was taken upon the lips of every Israelite and upon the lips of the early Christians. John MacArthur's denial of infant baptism is based upon a definition of faith that is essentially Roman Catholic and completely unbiblical.