The Answer of "who" is found by cross referencing John 8:51
Most assuredly, I say to you, if anyone keeps My word he shall never see death.”
The "how" is answered later in john 11:25-25
25 Jesus said to her, “I am the resurrection and the life. He who believes in Me, though he may die, he shall live. 26 And whoever lives and believes in Me shall never die. Do you believe this?”
So i think this is one place where the specific wording is very important, Jesus promises the believer will not "Taste" death. or "see" death.
Taste is used sometimes as a metaphor for "Intimate experience with".
So while Believers bodies will die, we will not "Taste" death. there will be no experience of "deadness" or "being dead". or as the old testament calls it, we will not be stuck "In the place of the dead". (sheol)
Just like the ancient Israelites we will be "Passed Over" by the Spirit of Death, having the Blood of the Lamb as our covering.
Also good to note that the Apostle Peter, in 2 Peter 1:16-17
16 For we did not follow cunningly devised fables when we made known to you the power and coming of our Lord Jesus Christ, but were eyewitnesses of His majesty. 17 For He received from God the Father honor and glory when such a voice came to Him from the Excellent Glory: “This is My beloved Son, in whom I am well pleased.”
So here, Peter, who knows he will die for Christ (John 21- a great reference for this topic.) Explains that he is an eye witness of the coming of Christ, he counts himself as already having seen the glory to come, because of the brief previews he got during the transfiguration, and the voice from heaven.