If my own experience as an atheist and agnostic is generalisable, many modern people find themselves stuck in a mid-ground - profoundly dissatisfied with the mainstream secular world view yet unable to believe Christianity because - by secular evaluative criteria - it seems arbitrary, uncertain and conflicted.
They are mentally paralysed by question upon question about the validity of Christianity, and even the definition of Christianity - life is full of questions and each answer merely leads to more questions!
They know that 'faith' is necessary; but this seems in the first place arbitrary, secondly, dishonest, and thirdly they are pre-convinced that it would be ineffective, since to know that everything depends on faith is itself eroding of faith!
So they are stuck - neither one thing nor the other.
You have a false and misleading model of how certainty and sureness is achieved. You have some notion that certainty is a product of logic and science - yet if you think further you will know that science is never certain and logic is controversial and errs (professional science and philosophy have been for centuries in a flux of change, at times overturning even the fundamental assumptions).
So it never has been and never will be possible to base you life on that kind of empirical/ rational certainty - it has never existed.
Social institutions - including churches - are human institutions, prone to error, prone to corruption.
The modern secular public realm is very obviously dishonest, manipulative, destructive - in a word evil. So, no help can be expected from them - but rather hindrance and subversion - from everyday human social interactions, and from the mass media, and from the official communications of social organisations such as government, law, education, health services, the police and military...
However, the mainstream Christian churches are also deeply implicated in this generalised corruption and destructiveness of modernity.
So the churches in general cannot be trusted, and the individual people who represent the churches cannot be trusted - and anyway, trust in any specific Christian church or person can only come after Christian faith, and not before it.
So, there is only one place on which certainty may be built, and from which certainty can be extended - and that is your own fundamental, innermost, bottom-line heart; your real self, which is the deepest sense you have of your own self.
Your real autonomous free self; inside and independent of your social interactions and the manipulations of human communications and pressures.
It is to this real self that you must turn for certainty - which you must ultimately trust.
At the same time, you know that this real self is hard to access, and it is hard to know for sure that you have understood and interpreted it accurately - that you are not merely expressing wishful thinking, or being manipulated.
1. Don't talk about your real inner self with other people; don't tell other people what you believe it says.
In a world of endemic corruption talking about your innermost discernible self will only increase the difficulty of locating and reading the real self; because of increased and more effective obstruction from the false self, the false self of corruption and manipulation, which is hiding your real self.
2. Accept on the one hand that your real self is the ultimate authority; yet that your understanding of your real self is both partial and prone to distortion.
Therefore, once you have decided that you understand something about the fundamental evaluation of your real self; you need on the one hand to accept it and build on it (since it is the best knowledge you have), while on the other hand you must be on the alert for feedback from your real self.
In this mortal incarnate life, you cannot avoid mistakes. The idea is that we act, because we must act; and often, inevitably and necessarily, we then repent our sins and errors to try again.
Your real self will tell you when you have made a mistake, a mistake about the evaluations of real self, or some other mistake in life.
But why should this real self be regarded the bottom-line, indeed why should it be regarded as valid at all?
Now, when you are a Christian you will know that the real self 'has God in it' - the real self is 'placed in us' by God for our wise guidance. And the Christian will have other sources to test and correct what we suppose to be the promptings of our real self - there is scripture, church authorities, church traditions and history... but these are not available to the pre-faith, pre-Christian, who is our current focus.
So what about the modern skeptic-atheist-agnostic who is stuck? Why should he trust that there really is a real self, and that this real self should be trusted above all other sources of knowledge?
My assertion is not that he should trust the real self, but that as a matter of fact - this is what he already does.
I assert that he already trusts his real self - but that the knowledge and insights of this self are very partial, very simple, unsystematic, personal - and therefore he allows them to be overwhelmed by the mass of public falsehood in which he is swimming.
I am urging that he should hold-fast-to and build-upon the knowledge he already has from his real self, and work from that - ignoring that it is small and simple and incomplete - and that it is contradicted by socially-accepted knowledge.
In the privacy of his own mind he should hold fast to that knowledge of the heart - no matter how tiny it may be - and place it above and beyond the mass of nonsense, falsehood, and manipulation that is the bulk of his thought.
If this can be done, then certainty and sureness and faith are the consequence and reward.
Start with a tiny glowing-coal of certainty and sureness - and it really does not matter how tiny - but here at last is something utterly solid, secret, secure and unassailable; from which, bit by bit, more and more may be built.