The Greek transliteration 'ou mē', translated 'never' in the majority of our English Bibles simply implies just that, 'certainly not, not at all, by no means'. Because of the differentiation in usage here, with the additions of 'for ever' & 'to the age', the idea that Jesus simply re-iterated what He had stated previously has been proffered; while this explanation may bear further examination & the burden of proof, it by no means negates beyond all reasonable doubt the fact that Jesus said what He meant & meant what He said!
So, what did Jesus mean? Clearly those who live ( in Him ) & believe ( in Him ) still die, or do they?! Was Jesus misleading His disciples, Martha in particular, telling them that those living & believing in Him would never die, biologically ( physically ) speaking, or was He alluding to the fact that they were more than the humanity that served to house their true Self, their Innermost Being?
The traditional ( orthodox ) Christian belief ( doctrine ) of 'the resurrection of the dead' seems to be predicated on the notion that physicality is at the crux of redemption. In other words, that even though one dies ( physically expires ), that one will not stay dead forever ( see Douay-Rheims, above ), but that at the Resurrection, which is misplaced into our future, their physical body will be reanimated ( returned to life ). This is the notion that Martha, like most Jews, had about resurrection. However, look at what Jesus said in response, leading into our central theme; 'I am the resurrection and the life': He told her that, in Essence, the Resurrection had come, that it was happening even then, both for her brother & others who had died, as well as those who were presently alive, into Eternity!
So, when Jesus spoke those words of comfort to Martha, He wasn't telling her that one who believed in Him would inhabit this biology forever ( that's really kind of ridiculous, unless one believes that Jesus wasn't the Resurrection ( as He said ), but would simply BE the Resurrection at some future date ). In other words, since Christians still die physically, He couldn't have been referring to biological life!
We have always known that we were MORE than just human beings, just what we see on the surface, or what we can view through a microscope! For a multitude of varying reasons, though, this knowledge, to a large extent, has been all but lost. No Christian will deny the fact that we are made up of 'spirit, soul, and body' ( I Thessalonians 5:23 ), but most will cling voraciously to the notion that our physical bodies are somehow not yet redeemed, though our souls ( spirits? ) are? Are there TWO Resurrections? Most Christians believe, as they must, that there ARE two separate resurrections, that Jesus ( yes, as THE Resurrection ) must needs come again to redeem this biology, in particular, our physical bodies: talk about a 'Gap Theory'!