Most Bibles translate this verse like this:
And as he sat upon the mount of Olives, the disciples came unto him privately, saying, ‘Tell us, when shall these things be? and what shall be the sign of your coming, and of the end of the world?’
The words marked in bold, however, could be spurious (fake) later additions for three reasons:
1. The parallel accounts in Mark 13:4 and Luke 21:7 do not mention these words. It seems absolutely incredible that words asking about ‘the end of the world’ (or, as more correctly translated, ‘the end of the age’), would be left out from the other accounts.
2. The words are out of context (like most fake additions). The question was prompted by Jesus saying that the Temple would be destroyed. Up to this point in the conversation, nobody mentioned Jesus coming in the future. Jesus later talks about a future coming in his answer, but there is nothing to indicate that the apostles had this in mind at the start. This could explain why no such words are mentioned in Mark or Luke.
3. The ancient manuscripts that we have for the gospel of Matthew are the most varied and corrupted of all Bible manuscripts, and we have none of any useful length from before the 4th century. What happened to the majority of the New Testament text before the 4th century is really a black box. Perhaps one day we may discover manuscripts that allow us to restore the original Matthew. But as of right now, we don’t have it. Therefore, any words that only appear in Matthew are automatically suspect. If anything else about them smells bad, then this increases our suspicion.
Therefore, we conclude that these provocative words could be later fake additions to the text.
Most spurious additions probably began life as a marginal note (or written between the lines) in a scroll used by a preacher. When scribes later copied the scroll, they didn’t know if the words were part of the original text or not. So, to play it safe, when making their copy, they moved the spurious words into the main body of the text.
This copy was then copied again, until eventually, these words became ‘part of the Bible.’ It’s not deliberate, just accidental. Such accidents are called interpolations.
Why, though, would a preacher scribble a note here saying “...of your coming, and of the end of the world?”
Well, in the years after Jerusalem was destroyed, some Christians were trying to work out why Jesus had not yet returned. So Matthew 24 was reinterpreted to have a secondary fulfillment beyond that of the 1st century. So by writing the words “...of your coming, and of the end of the world” near the text, a preacher would remind himself to explain this wider interpretation to his audience. Unfortunately, (if our suspicions are right) this eventually ended up in the text itself.
Presence
One of these possibly spurious words is a term usually translated as coming or nearness. However, the Greek word (παρουσία/parousia) is more correctly translated as presence.
Adventist denominations use this to argue that Jesus has already returned in an invisible presence. However, the Greek word was simply used to describe the royal visit of a king. The lexicon linked above says:
“used in the east as a technical expression for the royal visit of a king, or emperor. The word means literally the being beside, thus, the personal presence.”
There seems to be no reason to infer an invisible or spiritual presence, as its normal use is to describe visible physical events. Indeed, the same word is used in the Greek version of 2 Peter 3:12 to describe the Day of Judgment, not a period prior to it:
“...as you await and keep close in mind the presence of the day of Jehovah, through which the heavens will be destroyed in flames and the elements will melt in the intense heat!” –NWT (2015)
Therefore, what reason would anyone have had to think of it as invisible?
Also, the Aramaic text is quite different. In that language, the word in question simply means “coming”, but in the sense of the coming of a military force, in other words, an invasion. This would agree with the verse quoted above. So is there any possibility that the Aramaic-speaking Christians of the East thought that the Apostles mentioned an “invisible presence?” It doesn’t seem so.
Of course, all of this discussion is moot if the word really is part of a fake later addition to Matthew 24:3.