The main challenge is the denial of spiritual beings in any manifestation. If one does not believe in God, than one does not believe in Satan. Therefore one does not believe in Angels, Demons or spirits, or that man himself is a spirit.
From this basis, it makes it easier to deny any number of realities, including intelligent design of the planet, humans, animals, the environment or the independence yet interdependance of the anthropic constants of the planet, such as gravity, the amount lightning strikes that occur, oxygen levels, ratation of the earth, the axis of the planet etc. There are 122 of these constants.
Add to this mentality the aspect that there are roughly 4,200 religions in the world. The word religion is sometimes used interchangeably with "faith" or "belief system", but religion differs from private belief in that it has a public aspect.
What makes this a difficult proposition is the confusion that runs through so many belief systems. More importantly, everyone thinks that their belief is THE way to believe, and will defend their faith to the death in many cases.
From a spiritual perspective, this is a perfect strategy of Satan, to create so much confusion that people miss God altogether by either believing intellectually something that may not be what God has said, or becoming an atheist through an intellectual exercise bereft of any basis of faith.
Its a lot easier to check out and do your own thing than to get involved in what may be considered confusion and baseless principles in a diety they cannot relate to.
The problem with this position, is if their wrong…eternity is a long time to pay for such a choice….