Theology and Philosophy at their most basic are concerned with the metaphysics of Time or Change - and the perception that there can be no knowledge of anything without reference to that which does not change and is therefore Out of Time.
The foundational problem of philosophy is therefore the relationship between Time and Eternity, between Change and the Permanent.
Thus the Ancient Jews founded theology and the Ancient Greeks philosophy in the sense that the Jews perceived that the one creator God must be unchanging and permanent and inhabit an eternal realm Out of Time, while the Greeks perceived the same but starting at the opposite end.
Eternity does not change and is thus Out of Time.
However, this matter is absent from most religions, and from the minds of most people: most religions and most people conceive of eternity as endless Time: Time that goes on forever.
On this understanding, eternity is (merely) change that goes on forever: thus eternity is conceived as either open-ended and never-repeating evolution, or else eternal recurrence.
Within Christianity the concept of Heaven as Out of Time is apparently restricted to the Catholic denominations (and to relatively few persons within these).
Clearly, the concept of Heaven as Out of Time, eternal, unchanging - is not necessary to salvation; and perhaps the best of Christians do not even ask such questions as lead to theology and philosophy.
But wrong philosophical answers to this question lead, recurrently, to such errors and stumbling blocks as the debates over predestination and 'the elect' - matters which were conclusively answered near the beginning of Christianity (on the basis of Heaven being Out of Time) by Boethius, among others; but which nonetheless came back to do considerable damage at the Reformation and since.
And of course, every philosophical answer, no matter how correct and conclusive, leads onto further philosophical questions.
In this instance, philosophers ask how the eternal and the temporal realms are related; while Christians ask about the transition between Time and Eeternity which occurs at or around the time of death.
The Eastern Orthodox narrative of the soul guided by two angels through the upper airs, past toll houses inhabited by demons, for forty days before a first judgement as to its dwelling place - is one way of describing this which sticks to the concept of Time as linear and sequential. The Roman Catholic ideas of purgatory and limbo are other variations on the themes.
But none of the popular and accessible Christian accounts of what happens at death are able to say much about a transition between Time and Eternity (Out of Time); and indeed it is probably impossible to say anything more about this than Socrates/ Plato (who were probably the first to understand the problem).
In conclusion... if you have the kind of mind which sees the problem to which Eternity Out of Time is the answer - then you must not expect to get further precise and fully explanatory answer to the problems which follow on from this: the problems of the relationship and transition between Eternity Out of Time and Knowledge and this world we currently live in - this world of Time, Change, Decay and Illusion.