A great many atheists / humanists are essentially applying scientific process, and science is adaptable; it is entirely willing to change, as new discovery and observation is made. All you need to do, then, is to provide some kind of tangible evidence that actually stands up for more than a few seconds in the face of the appropriate community (so, the science community if such proof is scientific, etc). Such evidence has never been forthcoming so far.
A very good example of this is the current speed of light claims. Now, general accepted wisdom is that the speed of light cannot be exceeded; however, some scientists have submitted interesting results, with decent explanation of their method and workings, and have invited the community for feedback. The scientific community did not say "pfff you're wrong", but rather (paraphrasing) - "wow; that's staggering; that challenges an awful lot of existing findings - but hey, your process is not without merit - let's look into that some more". Indeed, regardless of the outcome, the science will have progressed for the better.
(Reversing that, the problem with religious dogma is that it generally cannot be changed, even when it is outright contradicted by science).
So: in answer to your question: find evidence. Any evidence of actual merit, that can be inspected, discussed, and validated or disproved (any genuine theory must be refutable - otherwise it is simply not a theory in the way that science means).
As another example; I currently have no reason to believe that fairies exist. I am aware that some people do believe in fairies, but my default position (given a complete lack of reason-for, or evidence-of, their existence) is that : fairies do not exist. However! If you can find reasonable evidence of fairy-folk, I'll be surprised and shocked, but I'll seriously consider it (by which, I mean I should also evaluate the merit of the evidence too; photoshop is rather too good these days, for example). By the same token, homeopathy - it has proponents, but also has a lot of counter-evidence about the subject. In many ways, the argument here is alarmingly similar to religion. To paraphrase Tim Minchin (by removing the rude bits advisory)
If you show me
That, say, homeopathy works,
Then I will change my mind
I’ll spin on a (redacted) dime
I’ll be embarrassed as (redacted),
But I will run through the streets yelling
It’s a miracle! Take physics and bin it!
Water has memory!
And while it’s memory of a long lost drop of onion juice seems Infinite
It somehow forgets all the poo it’s had in it!
You show me that it works and how it works, and when I've recovered from the shock
I'll take a compass and carve "fancy that" on the side of my (redacted).
A common answer here is that religion is outside of things like science; that is an acceptable answer, but in that case do not engage science with logic. We can always agree to coexist without trying to convert each-other.