The OP question is why doesn't God give us the answer?
Scripture does not deal with such matters.
Scripture gives numerous examples of God doing that very thing.
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Seems like it would help us out a lot if God told everybody, "The Calvinists are right" (Or whichever denomination).
So many false teachings would be disproven in an instant, everybody would know which Church best aligns with God & His word...
So why not?
Been there.. done that.
BTW I assume you do not mean that one denomination is saved and all others lost. Christ said in John 16 "I have many more things to tell you but you cannot bear them now" so then having less understanding of truth does not of itself make one lost. It is rejection of truth that does that.
I assume you mean "if God would come to Earth in the modern age and tell us " -- maybe in the form of some prophet?
Which of course groups like the Seventh-day Adventists claim He did that very thing.
And did that "solve the question" for "Everyone"?
No. We still have the divisions.
God told Noah that his religion was correct - but did that settle it for everyone else?
God told Moses and all Israel at Sinai that their religion was correct -- but did that settle it for everyone else?
God told John the baptizer that his religion was correct - but did all Israel listen?
Christ told His disciples that He was God the Son and that their religion was correct - but did that settle it for everyone else?
These supernatural events were indeed reported - but most people rejected that report.
Seventh-day Adventist will tell you of that same supernatural event within their own denomination - but that does not "settle it" for all who hear of it.
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Regarding the fact that the eschatology religion of Noah came direct from God - who tells Noah this is the right message you ask
When did God do that? Scripture please
Answer:
Genesis 6
13 Then
God said to Noah, “The end of all flesh has come before Me; for the earth is filled with violence because of them; and behold, I am about to destroy them with the earth. 14 Make for yourself an ark of gopher wood; you shall make the ark with rooms, and shall cover it inside and out with pitch. 15 This is how you shall make it:... 18 But
I will establish My covenant with you; ...22 Thus
Noah did; according to all that God had commanded him, so he did.
Genesis 7
Then the Lord said to Noah, “Enter the ark, you and all your household,
for you alone I have seen to be righteous before Me in this time. ...
Noah did according to all that the Lord had commanded him.
2 Peter 2
4 For if God did not spare angels when they sinned, but cast them into hell and committed them to pits of darkness, reserved for judgment; 5 and
did not spare the ancient world, but
preserved Noah, a preacher of righteousness, with seven others, when He brought a flood upon the world of the ungodly;
Genesis 6 -- for 120 years - Noah was given time to build the ark and preach.
Heb 11
7
By faith Noah, being warned
by God about things not yet seen, in reverence prepared an ark for the salvation of his household,
by which he condemned the world, and became an heir of the righteousness which is according to faith.