It’s a conclusion drawn on verse, not an actual fact.
The conclusion is
Enoch and Elijah.
Because neither died.
The verse in view is “And as it is appointed unto men once to die, but after this the judgment:” Heb 9:27
Now for the Spoiler (mine)...Ok, so if Heb 9:27 is telling us the 2 witnesses must have never died before and assuming the 2 witnesses are OT, then ergo it must be Enoch and Elijah.... and bob’s your uncle!
Except, what do you do with the NT people who were raised from the dead? (Chiefly Lazarus, Tabitha, Dorcus, etc). Well, those people died again (they are not here today right?) So out goes the supposition that Heb 9:27 is a RULE of God, rather it is talking about the general nature of man.
This is a prime example of how going PAST what scripture says actually can set you up for a big fall.
The real answer is less satisfying.....No person (other than God and perhaps his angels) know.
Moses and Elijah appeared at the transfiguration. The miracles described in Revelation align with those performed through Moses and Elijah.