You must be referring to the (at first confusing) verse:
Matthew 16:28 Truly I tell you, some who are standing here will not taste death until they see the Son of Man coming in His kingdom."
But look at what happens next after 16:28. Look and see:
28“Truly I tell you, some who are standing here will not taste death before they see the Son of Man coming in his kingdom.”
1 After six days Jesus took with him Peter, James and John the brother of James, and led them up a high mountain by themselves.
2 There he was transfigured before them. His face shone like the sun, and his clothes became as white as the light.
3 Just then there appeared before them Moses and Elijah, talking with Jesus.
4 Peter said to Jesus, “Lord, it is good for us to be here. If you wish, I will put up three shelters—one for you, one for Moses and one for Elijah.”
5 While he was still speaking, a bright cloud covered them, and a voice from the cloud said,
“This is my Son, whom I love; with him I am well pleased. Listen to him!”
Matthew 16:28-17:5
As you can see some (not all) of them witnessed Christ in His Kingdom, in Glory, and it happened 6 days after He said some would see this.
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Also, next, regarding the different aspect, the
2nd coming spoken of in other places (also before 16:28 in chapter 16 above) --> Peter, widely understood to be one of the main leaders among all the apostles, generally involved and central in their understandings and teachings, later would convey more via the spirit to all, on this question, to help them understand better:
2 Peter 3:8 Beloved, do not let this one thing escape your notice: With the Lord a day is like a thousand years, and a thousand years are like a day.
This fits perfectly to another key thing. A crucial thing, echoed in other places:
2 Peter 3:9 The Lord is not slow to fulfill His promise as some understand slowness, but is patient with you, not wanting anyone to perish but everyone to come to repentance.
God isn't it appears then likely to end this current world while many are being saved each day it would then seem(!) -- so long as more are seeking Him, trying to find Him in faith, He would not close the door it appears!
That seeking itself might eventually end some day, which would be a dark time, and then this continuing conversions which would seem to hold back the end would then no longer hold it back. It's a full-scripture kind of reading, where instead of one passage or one book one reads the entirety of scripture, and then these things stand out.
For you, you should stop trusting these kinds of objections made up by non-believers. They are meant to attempt to paint a picture on top of scripture, to repaint it's landscape into a new, different picture that is unlike the real, full text.