The items Key Piece #1 and Key Piece #2 increase the chances of finding an Angel Room instead of a Devil Room in 'The Binding of Isaac'. These pieces affect the percentages in which an Angel Room can appear instead of a Devil Room upon defeating a boss.
To answer your question, the item that increases your chances of finding an Angel Room instead of a Devil Room in the game The Binding of Isaac is Key Piece #1 and Key Piece #2. Both of these items are obtained by defeating one of the Angels that you can encounter in an Angel Room or by blowing up an angel statue.
Once you have either piece of the key, your chances of an Angel Room appearing increase. Specifically, having Key Piece #1 or Key Piece #2 increases the chance of Angel Rooms replacing Devil Rooms by 9.75% for Normal difficulties and by 15.75% for Hard difficulties.
The Book of Revelations does not directly increase the chance for an Angel Room, but it does increase the likelihood of Horsemen bosses appearing, which can indirectly affect the type of room appearing. Meanwhile, the Holy Mantle does not directly influence the chance for an Angel Room.
So, if you are looking to increase your odds of finding Angel Rooms in your runs, acquiring Key Piece #1 or #2 will aid in this endeavor.