As I have written (see references below): Christianity is incredible, and Mormonism is incredible-squared. In that sense it is perfectly reasonable to reject either or both - because there is no requirement for us to assent to the incredible.
However, rejecting Christianity and Mormonism simply because they are incredible makes no sense either - because that rejection itself leads to incredible conclusions.
To focus on Mormonism - it really is incredible that Joseph Smith (of all people!) should be a prophet of God and that the provenance of the Book of Mormon was as described (gold plates, angels, translating devices etc), and that Joseph's BoM translation really derives from a lost ancient manuscript.
So it might, superficially, seem straightforward to disbelieve these things. Let's call this the skeptical alternative. But what then?
Of course, most people who reject Mormonism as incredible have a rooted negative prejudice against it, do not know the whole story, and/ or they have wildly false or distorted ideas about Mormonism.
But if you approach the subject of Mormonism with a benign and sympathetic attitude, are honest and informed; then we can see that the skeptical alternative is also clearly incredible; because it requires on the one hand that Joseph Smith was both a genius and also a calculated fraud, who led a water-tight conspiracy; and furthermore that the CJCLDS grew from a foundation of fraud and conspiracy to become the (overall) highly positive and wholesome influence it is today.
But yet again, the skeptical alternative - while incredible and unprecedented - is not impossible.
It is possible to imagine or suppose that a fraudulent genius and a watertight conspiracy did indeed, by chance and against the original intent, lead to great good - why, not?
This belief goes against common sense and reasonable expectation, but it could be true.
But then, if we are honest and rigorous enough to apply this kind of negative, skeptical alternative reasoning to other domains of life - such as other religions, the history of politics, science etc.; then we will find that they also crumble away into what could be fiendish conspiracies.
In particular, we will be compelled to notice that Mormonism grew under the microscope of the mass media, and is vastly documented compared with other world religions and major Christian denominations.
We may reflect that absence of evidence is not the same as evidence of absence when it comes to these other religions, and that much of the 'evidence' about early Mormonism is both ignorant and dishonest - as well as rootedly hostile...
In the end, no matter where we turn, or how we twist and twist about - we cannot escape incredibilities: we really can't. Not if we are honest and rigorous.
The above is not by any means a proof of the validity of Mormonism - it is not intended as such, and I do not believe that there can be any such proof even in principle.
And what applies to Mormonism in small, applies to Christianity in large - the Christian story is packed with incredibilities and inconsistencies; yet to reject Christianity as incredible entails believing in some combination of delusion and conspiracy of a kind that is itself incredible, because grossly contradictory to actual behaviour and historical consequences.
It is that same skeptic's dilemma - Christianity could be based on delusion and fraud- that is not-impossible - it is just highly incredible. That path offers no escape from incredibility.
My point is that the impulse to avoid believing incredible things is mistaken, a basic error; because it is impossible. The notion of an incredibility-free belief system is an illusion and a snare.
Indeed, the urge to avoid incredibilities leads to the deep-rooted dishonesty and wilful self-blindness typical of the person who prides and advertises himself on being A Skeptic that relies only on Evidence.
(I mean the kind of man [we all know them - you may be one of them!] who applies skepticism only where and when it suits him, and blandly denies the incredibility of his own favoured incredibilities.)
But neither the skeptic not the credulous ever can or will avoid believing not just one but many incredibilities.
Incredible beliefs are simply a fact of life.
This can be taken in two ways - either as meaning that we cannot believe anything because we could believe anything; OR that this is the way things are meant to be - and that it is a necessary and desirable part of the human condition that foundational belief require an act of choice from each of us as individuals.
To believe that fundamental beliefs cannot and should not be wholly-dictated by objective public 'evidence' and 'reason' but necessarily require an act of personal choice is, of course standard mainstream Christianity - it is what is meant by Faith.
Skeptics assume that the only alternatives are either being convinced by conclusive and credible evidence to reach credible conclusions on rational grounds; or else just believing whatever incredibility you want and calling it 'faith'...
But Christians deny that these alternatives exhaust all possibilities, and also deny that the skeptical possibility is coherent (for the reasons given above).
So what should we do, each, as individuals? Does everyone have to believe in Mormonism because it is incredible, or because everything else is at least equally incredible? Obviously not!
The Mormon answer is that each interested person as an individual has the possibility of investigating the evidence - and each must (and inevitably will) then make a choice. But people should not believe in Mormonism unless that choice is validated-by, or indeed comes-from, divine revelation.
Evidence is relevant, but never conclusive. Each person who professes Mormonism needs to, and must have, faith.
And exactly the same ought to apply to any Christian denomination. To be any kind of Christian (rather than just doing things that Christians do) requires faith; and that faith is based on individual choice; and that choice - to be valid - is not arbitrary but divinely inspired.
Is this a process without any possibility of error? No.
Can we be sure and confident that divine validation has happened? Yes.
But might we then change our mind about things we used to be certain about, or doubt our own certainty? Yes.
Does this then mean that truth is relative and arbitrary and we can believe anything or nothing? No.
Truth is real, humans are fallible, certainty is possible, faith is necessary.
These just are the facts, and we must work with them - we have no alternative: we must choose, and we will choose and indeed we have already chosen (although not irrevocably).