Muizer,
When Noah and his family landed back on dry ground they already knew the Gospel and so they passed it on to their offspring who in turn passed it on and so forth. The sad part is that what they relayed became in many cases twisted making for other beliefs so God set aside Abraham that through him the true Gospel would be retained and its carriers the Israeli nation and out of whom the Christ would come which He did. So yes, you are correct in saying that a person can only become a Christian by the hearing or reading of the Gospel. The one sure thing is that even among the most isolated peoples the knowledge of there being a God was known by them.
Regarding the experiences of a religious nature for sure these events do happen and they happen because of the evil powers still at work even in our day. That will only change when Jesus Christ comes back to put all things in order including the end of the devil and his followers. No longer will he be able to convince people that God doesn't exist or that He doesn't mean what He says for on that day " Every knee shall bow and every tongue confess that Jesus Christ is Lord." To the religious He will say, " Depart from Me for I never knew you." That is what is written.
My experience is that I learned that man cannot become a Christian of his own volition, that it is God Who saves and that the Bible is His word as if He wrote it Himself warts and all. With this depleted body of mine the only strength I have left is the Gospel and being able to impart it as I do here.