I think what gives me the most doubts aren’t actually atheists but mostly
certain Catholics/Christians themselves, as bad as it might sound. For example Creationists and Catholics who take the Bible literally and try to convince you that the theory of evolution is a lie from Satan and/or the world is 5000 years old.
To me they make Catholics sound ignorant(I’m only speaking for myself). I find their opinions make me doubt more than those of atheists.
Also constantly hearing this and that is a sin, this will send you to Hell, this is wrong, if you don’t confess you will go to Hell, you have to live like this etc. is often overwhelming. Often Catholics have totally different opinions on the “truth” which is totally confusing.
I find this often really depressing. For these and others reasons I CAN understand atheists. Living without constant fear of going to Hell, end of the world prophecies, repenting, confessing and watching every step you do without feeling guilty,
seems a lot easier.
I think these might be some reasons why many people turn their back on the Church or start to lose faith.