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.