it seems the place this comes from is Revelation 3:20 (hovering should show the verse). but what does it mean to come in to (not into), and to whom is Jesus talking to?
anyway, long story short, the people he's talking to are already Christians, who are being lukewarm in the way they're living out their lives. and εἰσελεύσομαι πρός (eiseleusomai pros [enter in]+[to]) always has the idea in the NT of coming in toward/before someone/thing.
so what we can say is that it's not an offer of salvation, but a consequence thereof. and i would want to say it's a 'being with' idea, not 'dwelling within' - the verse continues with a shared meal, which would otherwise be parasitic!!! it's about fellowship with Jesus.
i was going to finish off this post talking about the fact that the holy spirit is who does dwell within the heart of the believer - but the closest i can find is 1 Corinthians 6:19 and 2 Timothy 1:14, but even they are inconclusive - especially 2 Timothy which could just as well be translated 'among us' (ἐν ἡμιν - en hemin) whilst 1 Corinthians talks about the Holy Spirit being in 'the body' of 'youse' - part of the theme of the picture of a church being one body, with Christ as the head.
where do we get this idea of inhabitation? sci-fi?
where else in the bible???
i guess if anything, it puts the emphasis back on salvation being relational.