"Faith" seems to take on a different meaning in this story. By definition, faith is a sort of trust or allegiance despite a lack of logical or physical evidence. In the case of Adam, there is textual information that seems to imply he has spoken directly to God. Usually, when God speaks to Adam and Eve, He uses angels as couriers of information. Seeing the angels also is evidence for God's existence. Furthermore, their expulsion from Paradise is more proof of God. In this case, then, what truly defines "faith?" Personally, I think it focuses more heavily on the "trust" aspect of the concept. There may be evidence for Adam and Eve to know that God exists, but that doesn't mean they trust Him or believe in His power.