To me, the challenges against the claims to the historicity, ethical coherence and relevance of Christianity begin with the points and questions listed below and build from there. If Christians are not able to establish reasonable (not exhaustive or perfect) answers to these issues, I don’t know how they can sincerely assert that their faith is true. It goes back to the adage of “extraordinary claims require extraordinary evidence”.
1. At least 300 million years of sentient animal suffering for creatures as large as rodents and dogs existed before humans arrived and inherited this situation. Horrendous pain was and is an intrinsic feature of the cosmos and biological life. Predation, disease, starvation, terror, depression, loneliness, birth defects, earthquakes, volcanoes, tsunamis, famine, hurricanes, meteors, tornadoes, etc. Further, predators were uniquely designed for killing other animals, just as certain kinds of diseases were designed to infect and diminish the health of living things. These facts directly contradict the idea of an all-good God who could have made a pleasant world and then humans caused the environment to change toward regular suffering and serious difficulties.
2. There is no evidence that a fall of humankind occurred.
3. There is no need for salvation unless one accepts the pure assertions of the Bible that humans are fallen from a perfectly healthy moral state at some time in the past for which we have no evidence or realistic description.
4. There is no realistic model proposed for how humans could function in a world without cell death (as this is how the Garden of Eden is often described, to various degrees). What we know as “human” includes things like a metabolism, between 50% and 90% of cells within our bodies being microbial (without human DNA) after birth onward, producing waste products that decompose, etc.
5. How is it possible, believable, intelligent, fair or wisely designed that one significant rebellion by the first humans would ruin the prospects for an inherently high quality of life for all future humans and seriously endanger them toward eternal torment unless they made the right kind of decision in response to a very confusing world full of suffering?
6. How is it possible, believable or fair that a good, competent, intelligent and wise God would make a cosmos and moral system that is so frail that it will collapse into radical disarray after just one significantly bad choice by two human beings?