Just about everyone hates and despises a "know-it-all", for basically two distinctive reasons, of which the first one is that the person that thinks that they know-it-all, fundamentally and conclusively does not; and the other reason is that those that think they know-it-all, actually do appear to pretty much know-it-all, but their arrogance and pride negates a lot of the value that others might get from their knowledge.
When it comes to the belief in God, there are all sorts of know-it-alls, of which by far, the worst of them all, is the person, typically of some intelligence, that just "knows" that God doesn't exist, and proceeds to pontificate on and on about why the existence of God is a delusion, or an opiate for the lazy and disenfranchised, and is prepared to do battle, on their terms, for any that dare to differ in opinion. There is always something extremely unsettling when intelligent people that really ought to know better, and really ought to investigate a lot further, instead, purposefully dismiss things of substantial substance as being a myth or a phantasm.
What makes it particularly unsettling, is that intelligent and very clever people, especially those that have self-serving morals and ethics, often do very well in rising up the ladder of business success, governmental success, political success, and material leadership of all sorts. Those that have a silver tongue and the Midas touch typically are going to be the ones that are the leaders in many a society, especially if their ambition is unbounded. So then, when those that are already in a position of authority, and are considered by most people to be credible, intelligent, and wise, then what they have to say and expound upon, can definitely make an influential difference in the lives of others.
So, quite frankly, the inconvenient problem with the belief in God, especially for those of blind ambition, is that God presupposes a moral universe, with natural law that is unvarying, and inalienable rights that all are gifted with, of which all of these things, explicitly mean that the measure of mankind is measured by an omniscient God, not by mankind. This means, that belief in God, necessitates that all of mankind is held to the very same invariable standard without an exception for anybody; as opposed to mankind, which like the changing of the wind, decides what shall go, and what shall be permitted; and what shall not go, and what shall not be permitted.
Those that are clever want to take the place of God, because that puts them in control, and will often provide to them the privileged ability to exploit all others in service to their specific needs and desires. Additionally, the power structure of many a modern nation wants to negate god, so that, that nation state in its structure becomes the replacement for God, or, in effect, supersedes and marginalizes God. Yet, again and again, no matter how clever the privileged few are that rail against the delusion of God; mankind, not now, not ever, will ever be able to kill God off, because all that ever is, and all that will ever be, is of God.
The vanity of vanities is the wrong headed belief that mankind made mankind, or, that mankind is just some sort of primordial happenstance. Those clever ones that claim that they know that this is true suffer from the darkest arrogance, which has made them delusional.