Ah, the great question of the church. Honestly, there isn't a clear answer; even in the Bible when people performed miracles, not all were doing it for God's sake, so even the claim that miracles will follow God's leaders is not entirely reliable.
Ultimately, this is my creed: seek truth above all else. Many men and churches and religious organizations will tell you what is right and wrong; who is to say whether it is you or they who are in the right? An open mind and a healthy dose of skepticism are necessary. Question authority whenever you see it; not in an accusatory manner, but in a manner that seeks to purge any and all flaws or mistruths for the benefit of all, and most importantly, question yourself and every last one of your own beliefs, and be willing to accept truths that may be uncomfortable or hard to swallow. Only then can you emerge confident in your beliefs. If you want to listen to the words of a deity, whichever deity that may be, don't listen to the words of men; go directly to the source. Then you will either be told what people you should listen to, or you will receive no answer and realize it is all a farce.
You have a question about God? Ask the sky. Whoever's listening will reply. It might not be in the way you expect or want, but either any higher power that exists cares about us and will listen to our pleas, or is uncaring or simply nonexistent. In any case, you will have your answer.
Truth will inevitably come to light. All men die, and in that moment, there shall be no arguing as to what, if anything, lies beyond. The truth does not change according to our perception; that is what the meaning of "truth" is. Therefore, if one wishes to have a solid foundation for one's life, truth is the most important thing to seek. As Mark Twain once said, "Get your facts first, then you can distort them as you please."
As your quote from Voltaire says, "Doubt is an uncomfortable position. But certainty is an absurd one." So be willing to doubt yourself and your most tried and true beliefs. I have done so, and slowly but surely I am emerging all the stronger for it, for like Descartes discovering his famous truth I think, therefore I am", we must find truth to base ourselves upon, lest all meaning be washed away in a sea of chaos and fluid perception.
"There lives more faith in honest doubt, believe me, than in half the creeds." - Alfred Lord Tennyson