The Greek word εν (en) simply means in. However, various Bibles have translated it as in, on, among, one, and in union with. Also, adding an apostrophe (or an ‘h’) to the word (‘εν – hen), slightly changes its meaning too.
The proper translation of this little word is important as it can greatly affect certain doctrines. For example, John 10:30 uses this word and the verse basically says, ‘The father and I are one.’ Now, some teach that Jesus was saying that he was one of three personalities of God in a Trinity. However, Jesus later used the same word (εν) at John 14:20 when describing his relationship with his disciples:
εγω εν τω πατρι μου και υμεις εν εμοι καγω εν υμιν
In that the day you will know that I in (εν) the Father of me, and you in (εν) me, and I in (εν) you.
Or, in our translation:
‘And in that day, you’ll come to know that I am in my Father, just as you will be in me and I will be in you.’
So if Jesus meant that he is the same person as (or is another personality of) his Father, then what he said at John 14:20 would mean that all of Jesus’ followers will become the same person as him and his Father; thus we would all become Jesus and God!
However, a more logical conclusion would be that just as Jesus and his Father are one in mind and purpose, the Christians will eventually join them in this perfect union.