Jesus plainly told his audience that some of the ones standing before him would not taste death before they would see the Son of Man coming in His kingdom. In other words, it would be in their lifetime. Jesus would come in His kingdom in their lifetime. That was in the first century. That is past not future. So, we can say for certain based on this verse that Jesus has already come in His kingdom and He did it in the first century during the lifetime of many who were hearing Him speak.