In the book of Hebrews we are told that Moses acted "by faith" in keeping the Passover (11:28). But why was faith necessary? I think that are at least three reasons.
First, it was novel. The prior nine plagues did not require the Hebrews to participate. Thus, faith was required because it this plague broke the pattern.
Second, faith was needed because it was "sacrificial" (pun intended). The Passover involved a costly sacrifice. Animals such as lambs were precious commodities, especially for impoverished slaves and to sacrifice an animal, your best animal (without blemish), would result in a significant material loss.
Third, observing the Passover required faith because of the magnitude of the miracle. Such a thing had never been done before. How could a few swipes of blood on the doorposts and lintel make the difference between life and death?