For me it was that the implications of atheistic materialism contradicted basic empiric knowledge. Atheistic materialism is an onthologic monism, the ultimate implication of it is that nothing can be defined since everything is a continuum. yet we can discriminate concrete objects and things. No essence can be defined in Mat-Ath, When I was an atheist I argued that the essence of things can be defined in their composition and geometry (arrangement) of constituent parts. But that is a weak argument since "objects" constantly lose atoms and gain new ones. Think about the atoms you lose and gain through all your life (throwback to the greek boat paradox)

Ultimately it was that in MatAth the person could not be defined, yet we are persons. Also the concept of specie was broken too, every animal would be its own specie.

Then I realized that atheists have no explaination for quantum probabilities, i thought that for God to not exist everything had to be explainable with mechanisms. But when we measure the spin of a particle, whether is spin up or spin down, there is a 50/50 perfect chance? what mechanism makes the choice? There is none, and atheists have no answer other than "thats just how the universe works, period" I realized that since there is no mechanism the only thing that remains to explain it is Will, and if there is will there is a person behind that will.

Almost every human being started believing in god(s) because they were introduced to the idea by their parents or other family members or teachers in stories and rituals and songs, usually long before they had the capacity to focus on objects more than two feet away. Every human society I’ve ever visited is saturated with the trappings of some religion or another, often many, and the messages and symbols of history’s many mythologies resonate in everything from the architecture to the money.

Those who don’t believe are usually the ones who have changed their minds, not the other way around. It’s not surprising that some number of those change their minds again.

Because of divine simplicity, the absolute maximum of perfection logically excludes the possibility of more than one.

if there were two gods, they would have to differ from each other in some way. But a being that is pure act (without any potentiality) and absolutely simple (not composed of parts) cannot have any accidental differences. They could only differ in their very “whatness” (essence). However, if they differ in essence, then one has a perfection the other lacks. The one lacking that perfection would not be absolutely perfect, and therefore would not be God. Thus, you cannot have two beings each claiming to be the maximum of being.

Occam's razor ("Entities should not be multiplied beyond necessity")?

I mean, God, isn't one enough? Honestly, it's too much for me!