I was a lifelong Christian, it was a careful reading of the Bible that began the process of shedding my religious beliefs. There are now 100's of reasons I can give that all point me in the same direction but I will focus on the big three here.
In short the Bible ordains slavery (and reaffirms this position in the NT), commands genocide (the Amalekites and the Seven Nations), and has God committing infanticide (slaughtering all the first born of Egypt).
Slavery: see, Slavery in the Bible
Apologetic: Slavery, Does God Approve of It?
Note how the above Apologetic version fails to mention key passages where the non-Hebrew slaves can be kept forever, is treated as chattel, and can be beaten.
For example it mentions Leviticus 25:39-43 - the Hebrew "indentured servitude" passage but excludes the relevant passage is 44-46 - this is a disgusting display of dishonest non-scholarship.
BTW: indentured servitude is also a blight on our history so appealing to this horror doesn't make things any better. But it's also wrong because it's not 39-43 in question, it is 44-46. [they also ignore how you can trap even a fellow Israelite into full slavery]
The dishonesty of much of Apologetics quickly turned me off trusting them and doing my own research.
It is also is irrelevant how 'nicely' they supposedly treated those slaves that they could beat so long as they didn't die in a day.
If you are using these diversions to excuse your Bible you have already lost the moral high ground that you suppose God to be.
For me, slavery is a categorical wrong - there are no versions of it that are tolerable.
For me the position was untenable. I tried to deny it for a long time but eventually, on careful and extended study, it became clear that Bible ordains slavery and that this was not a position I could hold.
1 Samuel 15This is very clear and unambiguous command to genocide an entire nation, and not content to stop at children and infants, slaughter their animals as well.
2 This is what the Lord Almighty says: ‘I will punish the Amalekites for what they did to Israel when they waylaid them as they came up from Egypt. 3 Now go, attack the Amalekites and totally destroy[a] all that belongs to them. Do not spare them; put to death men and women, children and infants, cattle and sheep, camels and donkeys.’”
Apologetic: Slaughter of the Canaanites | Reasonable Faith
Note that William Lane Craig appeals to you to feel sorry for the poor soldiers who had to mass murder all the women and children. Craig sufficiently demonstrates the horror here that I need no rebuttal.
Exodus 11Are we supposed to feel sorry for the Holy Spirit this time?
4 So Moses said, “This is what the Lord says: ‘About midnight I will go throughout Egypt. 5 Every firstborn son in Egypt will die, from the firstborn son of Pharaoh, who sits on the throne, to the firstborn son of the female slave, who is at her hand mill, and all the firstborn of the cattle as well. 6 There will be loud wailing throughout Egypt—worse than there has ever been or ever will be again. 7 But among the Israelites not a dog will bark at any person or animal.’ Then you will know that the Lord makes a distinction between Egypt and Israel. 8 All these officials of yours will come to me, bowing down before me and saying, ‘Go, you and all the people who follow you!’ After that I will leave.” Then Moses, hot with anger, left Pharaoh.
9 The Lord had said to Moses, “Pharaoh will refuse to listen to you—so that my wonders may be multiplied in Egypt.” 10 Moses and Aaron performed all these wonders before Pharaoh, but the Lord hardened Pharaoh’s heart, and he would not let the Israelites go out of his country.
The sickness here is that you must imagine some morally sufficient reason for your God to murder all the first born of Egypt in order to make his point and believe that no other method an all-powerful God could have used would have sufficed.
Oh it's ok, all the children get to go to heaven - right?
Do you feel the same way towards Andrea Yates?
If not, why not?
God commands Abraham to sacrifice his own child and what does he do? He loads up the donkey and heads up the mountain to do it - so clearly Abraham believes in a God who would and could command a child sacrifice and expect it to be carried out. But somehow you have special knowledge that God wouldn't do this to Yates?
How about this -- if you hear God commanding things in your head you get professional help immediately, regardless of what that voice is saying.
So these are all the kinds of things that put the seeds of doubt in my head and caused me to question and search for a decade after that (during this period I mostly considered myself Spiritual But Not Religious). I had a strong bias inculcated in me against 'Atheists' so I didn't really read anything by atheists until much later.
I studied many religious traditions and I studied a lot of science - physics, cosmology, neurology, biology, mathematics, computation, etc.
Hindus have faith in their beliefs, Sunni Muslims have faith in theirs, Shi'i have faith, each of 30000 sects of Christians have faith, etc... And some of those beliefs directly contradict each other. So Faith is an unreliable methodology that seems to land a person in the faith in which they were inculcated a very large fraction of the time. This is clearly not a path to truth and neither is the oft heard appeal "I haven't been proven wrong" - this is a classic logical fallacy: argument from ignorance. This is the fundamental error of Faith.
I've also had 'personal experiences' so I also find those to be unreliable when not properly questioned and studied and held up to the same standards as we require from other areas of study. Brains create nonsense all the time - for example dreams - and I have had 'Lucid Dreams' and dreams while I was awake. Brains are clearly demonstrated to be unreliable when in altered states of consciousness.
Finally it came down to an observation by Hume, and echoed by many others (I first encountered the idea in T Huxley) -- proportion belief to the evidence, and evidence to the claim.
If a God exists it doesn't seem to want us to have a reliable method of knowing about it.
There are two mysteries that keep things interesting (for me):
Qualia - why we seem to have a conscious experience
Existence - how does stuff exist
Neither of these are sufficient to appeal to a God and certainly not sufficient to appeal to a specific murderous deity.
Just because a concept is imagined to explain some phenomenon doesn't mean that concept is true or offers any actual explanation. Invisible Pink Unicorns that fart universes into existence could "explain" Existence, Qualia, rainbows, and why we don't see them - this is NOT evidence for the claim. This and God is a superficial type of "explanation" that doesn't actually explain anything but rather just substitutes itself for the original. When you think that God explains the origin of existence, life, morality, etc you are committing this grievous error.
Once this is understood the hollowness of the concept becomes readily apparent.