Hi, Truly.
There are times when the same terminology is used in the Bible (like "begotten," for example), but the intended meanings are different, and that is why context is so important. In this response, I will only focus upon what Jesus said in John 8:51.
Jhn 8:51
Verily, verily, I say unto you, If a man keep my saying, he shall never see death.
What type of "death" was Jesus speaking of here?
Was he speaking of physical death?
Was he speaking of what we might call "spiritual death" or being "dead in trespasses and sins" (Eph. 2:1, Col. 2:13)?
Was he speaking of what the Bible calls "the second death" (Rev. 2:11, 20:6, 14, 21:8)?
Obviously, he was speaking of "the second death."
How can I/we be sure of the same?
Easily.
For one thing, we can compare what Jesus said here to what he later said to Martha.
Jhn 11:21
Then said Martha unto Jesus, Lord, if thou hadst been here, my brother had not died.
Jhn 11:22
But I know, that even now, whatsoever thou wilt ask of God, God will give
it thee.
Jhn 11:23
Jesus saith unto her, Thy brother shall rise again.
Jhn 11:24
Martha saith unto him, I know that he shall rise again in the resurrection at the last day.
Jhn 11:25
Jesus said unto her, I am the resurrection, and the life: he that believeth in me, though he were dead, yet shall he live:
Jhn 11:26
And whosoever liveth and believeth in me shall never die. Believest thou this?
Jesus told Martha that although believers in him were dead, as in having partaken of physical death, they yet would live, and never die. The "never die" part, in context, pertains to the resurrection from the dead, and this is what we read about it in the book of Revelation.
Rev 20:6
Blessed and holy
is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years.
The saints of God will be partakers of "the first resurrection," which coincides with the time of Christ's second coming (he only comes twice - Heb. 9:27-28), and "on such the second death hath no power." THIS is the "death" that Jesus was speaking of in John 8:51.
With the exception of the saints who will still be alive at the second coming of Jesus (1 Thes. 4:15-17), all Christians will eventually partake of physical death, so Jesus could not have been talking about physical death in John 8:51. The only other options are "spiritual death,' as in being "dead in trespasses and sins" (Eph. 2:1, Col. 2:13), and what the Bible calls "the second death" (Rev. 2:11, 20:6, 14, 21;8). When it comes to "spiritual death," everyone has partaken of the same, so we can easily remove that from the list of what Jesus intended to convey. That only leaves us with "the second death," and no Christian will partake of the same, so it is a perfect match.
Lord willing, I will add another post later today that will bring even more clarity to this truth.