Firstly a set of abstract rules would not give rise to anything.
How do you know? We are talking about the reason for existence, nobody actually
knows anything at all about it.
Secondly Feser is not trying here to say that this argument explains Christianity but theism. That it scotches the assertion that belief in God is unreasonable. This he makes clear in his blog. Trying to strip God of his traditional theological attributes in an act of historical revisionism is a standard antitheist ploy.
The argument is for theism not Christianity.
The point is that as soon as you add the characteristics of the Christian god - or any other interventionist god that interacts with its creation, makes moral judgements, answers prayer, and generally displays any characteristics of a thinking mind - you make Feser's argument self-contradictory.
You can't accept Freser's argument
and believe in the Christian god. If his argument is valid (which it isn't - see below) then it would be an argument for deism not theism. Theism contradicts the argument.
Finally you say Tegmark is platonic. Early Christian philosophy has a huge neo platonic component in it.
You cannot again allow one thing in Tegmark and announce its conclusions as a vice in Christian thinking.
Once again, I'm not doing that, as I've explained several times.
We can actually use Tegmark to refute Feser. All you need to accept is that Tegmark's notion that we are actually in a Platonic mathematical structure (because all such structures exist) is a logical
possibility. It doesn't need to be true or even probable, just
logically possible.
I stand to be corrected but I can't see a way to logically rule it out.
We can then refute Feser using a
reductio ad absurdum.
Remember that Feser is supposedly deducing the
necessary characteristics of his 'end of hierarchy' without reference to what it is, so if mathematics can logically fit into that role, his arguments should apply to mathematics.
We do indeed find that some of his characteristics do fit (with the Platonic view of mathematics) - independent of space and time and so on. However, his supposed arguments for thoughts, intelligence, omnipotence, and omniscience should also apply.
This is plainly ridiculous. Mathematics doesn't know anything at all, it has no will, so cannot exercise any power at all. It plainly cannot think and is not intelligent in any way. It's a set of (necessary, in the Platonic view) logical constructs.
Therefore, Feser's arguments for these properties are invalid because they clearly do not apply to
any possible 'end of hierarchy'; mathematics is a counterexample.