It's very inspiring for the people, like Ariane to want to prepare and pass special examinations which would give them a special certificate that they are qualified to be liturgical organists. It gives a great sense of focus because now it's no longer just a dream, it's a likely outcome.
If you also want to pass the exams in your country, obviously the first thing is to build up a little bit of liturgical repertoire (at least some 12 pieces) - preludes, postludes, pieces for communion, and offertory, wedding processionals and marches, as well some funeral music. You would also need to be able to accompany hymns and psalms. In many cases, sight-reading, transposition, harmonization and basic hymn improvisation skills will be required, too.
It is frustrating to watch yourself play and be quite unpredictable in terms of quality - mistakes in easy places or mistakes in difficult places or sometimes no mistakes at all. What does it all mean?
I think it means you need to work even more diligently on getting the details perfected. Do you practice finger and pedal preparation repeatedly ? If not, you should be. This action which allows you to slide your foot with one swift motion into the new position for the next note and wait it here practically automates your pedal playing. The same can be said about the finger preparation, especially in leaps and places where you have to switch position. Check if you are depressing the pedals with the inside of your foot. This greatly adds to the overall precision of pedal playing.
Use this time wisely, even if you have just one hour available, like Ariane. It's not too little to start seeing the results you want. If you still feel frustrated and doubtful it is because probably you are uncertain whether or not you are using this time effectively.
Let me put it this way - it's way better to work on a couple of pieces in a deep level in this hour than to jump from one piece to another without actually achieving anything of value (unless you are sight-reading which has entirely different purpose).
Memorization is a challenge for a lot of people. Even teachers often don't tell us how to do it. They say - memorize this piece or a page of this piece and bring it to me next week. That's not enough to start to feel at home and secure and calm when learning to play without the score.
Regardless of what method you choose (voice by voice, like Helmut Walcha liked or measure by measure like Marcel Dupre recommended) here's what's really crucial - don't play the piece over and over hoping that one day you will learn it to play by heart. Yes, you might, but it's unpredictable - in time of stress when you have to play in front of other people, you might forget some crucial sections which will ruin your performance.
The more ruthlessly systematic you become in learning to play by heart, the longer you can keep this music in your long-term memory.
Supply the tenor part to the above excerpt which is taken from my Processional March in C Major (try not to look at the answer ahead of time).
No. 5 Der Winter will hin wichen (p. 28) from Buxheimer Orgelbuch (ca. 1450), a German Renaissance collection of organ music.
All Depends On Our Possessing