How about a slightly different answer: The creator deity did not actually give them magic proper.
All the magic circles, runes, components, chants, and other items are basically a mystical programming language that the creator deity gave to their chosen race to enact miracles up on the land. Humans are constrained to whatever this magic language can do, and while we can discover better or more efficient uses of this language, there is a limit to what it can do. The clergy of the creator deity extol this divine gift and their deity's love and promise good things for following this dogma. In addition, they actively discourage anybody from finding out the truth about magic.
A hidden reality could be that this deity gave a constrained magic both to prevent humans from rising to the level, and thus being able to challenge, the gods as well as syphoning tiny fractions of every spell to increase their own power. Nothing overt, but enough fractions of a cent can make dollars in the long term, and who will have the power to challenge them if they are found out?
The Dark Ritual(TM) is a twofold thing. First, it is the discovery of the actual progenitor/source of magic itself. Second, it ritualistically breaks your dependence on the magic language to get results and allows the user to cast directly.
The first part is why it can only be done once -- the first time, you break the bond to the magic given to you by the creator deity. To perform the ritual again breaks your bond to magic itself, rendering the user mundane or worse.
The second part means that all the steps that you went through physically before are now done magically. This does not necessarily make you stronger at the outset, but it does make you faster at magic from Day 1. In the beginning, this is a serious advantage and will enable you to survive long enough to get stronger. Doing everything yourself depletes yourself faster which leads to yourself getting stronger so you can keep up with the toll put on your body.
Since you are putting a greater overall toll on your magic, it will strengthen faster. This is why those that have broken the bond are stronger -- practice.
There can be few reasons as to why this is a Dark Ritual(TM). Pick and choose at will.
- Undertaking it labels one a enemy of the Church and will lead to a Very Painful Death if caught.
- Some die due to overexertion in the first days from casing spells that were within their power level before but not within it now that they have to do everything themselves
- Some heed the warnings of a cost or sacrifice that must be paid to know the Truth and do not undertake the ritual. Likewise, others end up sacrificing too much to learn Truth and suffer horribly if not die outright.
- Undertaking the ritual exposes yourself to side effects that the magic language prevents. Some are good while others aren't.
- Knowledge of the ritual is not constant. This can be for a variety of reasons
- It is placed by the true patron of Magic to be found by the people that they deem chosen ones and able to handle the responsibility
- Inversely, it is given to those it has deemed will kill themselves by using it as a punishment or karmic justice.
- A third option is that it is given to people that will make the world more interesting. Your definition of interesting may vary.