Being a Catholic I obviously don't believe in
sola fide; however, the idea deserves a little more thought than it's been given here. For starters, most Protestants don't literally mean simply to say a prayer and it no longer matters what you do. Instead they think that if you really accept Jesus in your heart, you
will do the right thing anyway, and that part is of secondary importance at best. Many will also argue that if you don't do the right thing afterwards, at least for the most part, then you never really believed in the first place.
Others will take a more nuanced approach and say that faith is not a single event in time but a continual state of saying yes to God. This last definition comes close to the Catholic understanding of grace as something that can be lost through sin. We also believe it is unmerited on our part, but that we can reject it through sinning. Thus the Catholic understanding is not simply that we must believe but that we must also change our lives to act in accordance with that faith. An astute individual will notice that depending on the precise definition of these terms, the difference between Catholics and Protestants may be very small in this regard.
It is also worth defining "faith". The traditional understanding of faith is a rational act of the will to place one's trust in another or accept the testimony of a witness and take them at their word. So, for example, when I get on a plane I place faith in the pilot to be able to safely fly me to my destination. That faith is rational; the pilot is licensed and flying for a commercial airline, so I have reason to think I should place faith in the pilot. But it is still placing my faith in someone beyond myself and what I can conclusively derive with my own intellect. Faith is not simply a belief in the existence of an entity.
