However, there is a relatively simple yet powerful procedure in memorizing the piece. So here are my recommendations on how to learn organ pieces by heart.
Practice your piece in fragments. Take a fragment of four measures and master each individual measure. Play this measure five times when looking at the score and five times without looking at the score. Do this for each of the four measures in your fragment.
Then go back to the beginning of the measure and practice two measures at a time - measures 1 and 2, measures 2 and 3 and measures 3 and 4. After that play three measures without stopping - measures 1, 2, and 3 and measures 2, 3, and 4. After that, memorize measures 1 through 4.
If you do this for every fragment in your piece then you will have no trouble in memorizing it. You have to spend each day perhaps 15 minutes with each fragment and remember to repeat previously mastered fragments at the beginning of your practice session.
So little by little, fragment by fragment you will memorize the entire piece. Then you go back and practice two fragments or eight measures without stopping. When you reach the end of the piece, then you practice four fragments without stopping and so on.
You always have to double the length of the fragments. So finally you will reach the level when you can play the piece by heart from memory without looking at the score and without stopping fluently.
So you see how it is simple to memorize organ pieces in this step by step manner - learning fragment by fragment and then adding one fragment to another. Apply my tips in your organ practice and then you will be able to play organ pieces by heart easily.