So when he said in Matthew, "Verily I say to you, there are certain of those standing here who shall not taste of death till they may see the Son of Man coming in his reign." was he saying he would reign in their minds?
When scripture refers to the coming of the Son of Man, it is referring to his literal, physical return. Christ' reference, "...won't taste death..." before his return means that those who believe in him will not taste spiritual death which is the true meaning of death, a total separation from God.
As Christ said to the thief on the cross next to him as they were being crucified/executed;Jesus answered him, “Truly I tell you, today you will be with me in paradise.” Luke 23:43
In that passage it is obvious that physical death will occur for them but Christ is inspiring when he indicates that their spirits will continue to live.For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord. Romans 6:22-23
In that passage it clearly states that death occurs because of sin. This means a total separation from God. Then the scripture assures that God provides a gift, an alternative to death which is eternal life. This again is referring to one's spirit which will live on for those who accept the gift. Because in order to have a gift, one must first be offered one and then accept it.