Use 50 miracles.
Upon unlocking a miracle you will be given 3 uses of it. This will allow you 21 free uses of your miracles overall. You will then have to purchase more through the hub to use the remaining 29 miracles. Touch the bible icon in the top left corner of the screen and then touch the miracle you wish to purchase more of. Touch the dollar sign in the green box in the bottom left corner of the screen to purchase one at a time. The prices for another miracle use costs $1,250 to $5,000. For $25,000 you can purchase a third miracle slot. The purchase option is found on the same page as you find the miracles on only this time the dollar sign on the lower right side is the one you want to touch.
If you are in the middle of a mission and wish to purchase more miracles without having to go back to the hub, you can purchase more from the pause menu as well. Touch the pause icon in the top right of the screen from the mission map then touch miracles to bring up a new menu. A list of the miracles you have equipped will show up. Touch the dollar sign beside it to purchase more stock of the one you want. You can only purchase more for the ones you have equipped. Miracles that aren't equipped must be purchased from the hub.
Make sure the miracles you want equipped have a small green check mark on their icons before returning to battle. Once you are back in the mission map or battle screen, touch the bible icon in the bottom left corner of the screen to pause the game action and bring up a square in the middle of the screen. Use your finger to draw the symbol shown under the icon of the miracle you wish to perform inside the square. If drawn correctly, the line you leave behind will turn into the icon's image and activate.
The total is cumulative between all miracles so simply use any one 50 times and the trophy is yours.