The revelation of God. That’s what faith is: believing what God has revealed based on the authority of the revealer. Science, on the other hand, helps us obtain knowledge about the natural world that we can sense. But science cannot prove, for example, the nature of the Trinity, the truth of the hypostatic union, that if we obey the commandments, we will be saved, that our sins our forgiven in the sacrament of penance, etc., etc. We only can know these to be true because God has testified that they are.