First, some have suggested that this may be a reference to the Day of Pentecost where Christ’s Helper, the Holy Spirit, came to descend upon the apostles. In John’s Gospel (14:26), Jesus promised to send the Holy Spirit, and, in the beginning of Acts (1:4–8), He tells them not to leave Jerusalem until they have received the Holy Spirit. But this hardly seems to fit the description of seeing Christ coming in His kingdom (Matt. 16:28).
Second, others believe this might be a reference to the destruction of Jerusalem and the temple in a.d. 70. This would mean that He would return to bring judgment upon the city that rejected Him and crucified Him. While this is a possible explanation, it does not seem to account for the fact that Jesus appears to be coming for believers (those “standing there” with Him), not simply coming in judgment on unbelievers. Nor does the judgment on Jerusalem in a.d. 70 adequately express seeing the “Son of Man coming in His kingdom” (v. 28), a phrase reminiscent of His second coming (cf. 26:64). Nor does it explain why Jesus never appeared in a.d. 70.
A third and more plausible explanation is that this is a reference to the appearance of Christ in His glory on the Mount of Transfiguration which begins in the very next verse (17:1). Here Christ does literally appear in a glorified form, and some of His apostles are there to witness the occasion, namely Peter, James, and John. This transfiguration experience, of course, was only a foretaste of His Second Coming when all believers will see Him come in power and great glory (cf. Acts 1:11; Rev. 1:7).
Geisler, N. L., & Howe, T. A. (1992). When critics ask : A popular handbook on Bible difficulties (349). Wheaton, Ill.: Victor Books.