Many preterists (full or partial) hold to the two-age model of eschatology (the Jewish age ended in 70 A.D., ushering in the Gentile age). How do preterists reconcile this perspective with Jesus' promise in Matthew 28:20 that He will be with Christians "all the days until the completion of the age" (literal translation). Does that mean that Jesus is no longer with us since the age to which He was referring has ended from a preterist perspective?
Although I'm not a preterist (none of that is important to me), this is the argument you would need to consider.
Teaching them to observe all things, whatever I have commanded you, and behold, I am with you always until the end of the age. Amen.
διδάσκοντες αὐτοὺς τηρεῖν πάντα ὅσα ἐνετειλάμην ὑμῖν καὶ ἰδού, ἐγὼ μεθ᾽ ὑμῶν εἰμι πάσας τὰς ἡμέρας ἕως τῆς συντελείας τοῦ αἰῶνος ἀμήν
It is natural for us in English to think of the conjunction ἕως (heōs), translated as "until," as the terminus ad quem of an action.
That is to say, most read this scripture and think, "Jesus will be with the disciples until the end of the age, but after that, he will no longer be with them." In other words, the "end of the age" is the terminus ad quem or "limit to which" Jesus will be present with them, and after the end of the age, he will no longer be present with them.
However, ἕως is a bit more flexible than simply pointing to the terminus ad quem of an action.
Consider 2 Samuel 6:23.
And Mikhal, the daughter of Sha'ul, did not have a child until the day of her death.
καὶ τῇ Μελχολ θυγατρὶ Σαουλ οὐκ ἐγένετο παιδίον ἕως τῆς ἡμέρας τοῦ ἀποθανεῖν αὐτήν
Mikhlal did not have a daughter until she died. Quite plausible, possible, and probable.
However, if ἕως points to "the day of her death" as the terminus ad quem, then we must admit that Mikhlal had a child after she was already dead.
Such is impossible. Clearly, ἕως does not always point to a terminus ad quem. Mikhlal did not have a child to the day of her death, and thereafter. Likewise, Jesus would be present with his disciples to the end of the age, and thereafter.
Not necessarily. Technically one could take the two-age model literally where one age would end in 70 (and thus Jesus would no longer be with us), but most who hold to this believe that the ages overlap. So Jesus was talking about the present age that overlaps the former age when he said it. Thus he will be with us to the end of the current age, which overlapped the former age.
Good question. As a Full Preterist I would respond "No". Some nuances can be lost in translations and this is a good point. If my child were facing a surgery I might try to comfort them by saying "I will be here for you, even through this end of this surgery". I'm not saying after the surgery I am suddenly leaving them. The major point is I will always be there for my child, even through this painful part of their life. Yeshua established the major point in the first part of the statement then followed with an example of his faithfulness. The other part of the statement that should prove his ongoing relationship is context. In this context, the typical translation, albeit misleading, is "end of the world". This has lead many dispensationists to point at a non-point. Translation is "completion of the age". The Jews understood two ages: This Age and The Age To Come. They were living under the Mosaic Age and looking forward to The Age To Come. They had total understanding of what Yeshua was telling them: that even after the transition of the "Ages" Yeshua would still be there for them. Not a problem for them but can prove to be an issue for believers living under different cultural conditions 2000 years later. One thing to consider about the middle 15th-16th century, their means of communication varies a lot from how we speak and understand each other. Their particular explanation of "heos" might have been "through" for us. Therefore, the verse might as well read "I will be with you all the days, even through the end of the age".