That is a great point. The Bible does not talk about this topic of regeneration at all. It is s big leap and assumption on their part.
You can't say not at all. That simply isn't true.
Our English translations use the word "regeneration" only once that I remember and the context really doesn't give us any hint of what event or events are referred to.
Genesis or generate means the beginning, in a generally accepted definition. Regeneration would mean "beginning again".
Some use the word to be a virtual synonym for the term "born again". Others use it to refer to the very beginnings of the salvation process.
It is this later use that is common for Calvinists. They are talking about that special act that brings to life the dead spirit of fallen men and allows them to commune with the Holy Spirit. It is seen that way because spiritual death is usually defined as man's spirit being disconnected with the Holy Spirit - unlike the way it was in the beginning when men communed with God.
It's really not a big leap to say that there had to be a beginning point of our spirit being reconnected with God's. The problem seems to be how to determine
when that takes place. That's where the assumptions come in -
for both sides.
Calvinists see it happening before the gospel is heard - which allows us to understand and respond to the gospel in a saving way. The opposite side of the issue seems to see that reconnect as a kind of "reward" for our coming to God and believing the gospel of our own free will even as spiritually dead and fallen men.
Somewhere in the middle is what I would call the scripturally bankrupt view that God regenerates every man somewhere along the line so that they can hear from the Holy Spirit. Granted - that's a rather rough description of so call "prevenient" grace. But I think it's a fair one. Most of those folks wouldn't put it the way I did. But then most of those folks understandably are really vague about exactly what they see happening with prevenient grace. It seems to me to be a way to have their cake and eat it too.
Although I am not as dogmatic about exactly how this takes place as most Calvinist types - I find their view more scripturally accurate than the ones of the opposite sides.
What evidence do you have that there is a regeneration before belief. Concrete verses. Not double talk or special interpretation.
Because the word is only used once and then in a rather vague way, it is not possible to show an exact verse that teaches that it happens either the Calvinist way or the other ways. This is especially true since there seem to be two views about what the word means.
It isn't just the Calvinists who would be hard pressed to prove their case with concrete verses.
I don't want to muddy the waters too much. But I actually hold a third view that sees regeneration as more of a process than a particular spectacular event. Of course this denies some of the teachings of Calvinists on total depravity and irresistible grace. And, of course it smacks of special grace just as does the Calvinist's view. So it would be rejected by non-Calvinists out of hand. So I won't go into it more here except to say that it also agrees that there had to be some point of beginning for spiritual life. I just wouldn't venture to say when that might be for any given person. I will say though that it seems to me that it has to happen before the exercise of saving faith.
As with doctrines like the Trinity and a few others - the doctrine of regeneration is more of a systematic arrival at truth than an outright single statement of doctrine directly quoted from scripture.
Before you say that it can't be a valid doctrine under those circumstances - realize that you would then have to throw out a great many teachings that we all subscribe to. Much Christian truth has been arrived at through a systematic combining of scripture with scripture - which is IMO the way God intended it to be.