Detailing how that is done is the precise purpose of this guide, and fans that read on should have no trouble entering the mirror in The Binding of Isaac: Repentance and obtaining the game's first Knife Piece. Specifically, this mirror makes its appearance in Downpour 2 and Dross 2, and indeed it is possible to pass through it.

As a player makes their way through The Binding of Isaac: Repentance, they will ultimately encounter a mysterious mirror.