The Greek word a·tha·na·siʹa is formed by the negative prefix a followed by a form of the word for “death” (thaʹna·tos). Thus, the basic meaning is “deathlessness,” and refers to the quality of life that is enjoyed, its endlessness and indestructibility. (1Co 15:53, 54, ftn; 1Ti 6:16, ftn) The Greek word a·phthar·siʹa, meaning “incorruption,” refers to that which cannot decay or be corrupted, that which is imperishable.​—Ro 2:7; 1Co 15:42, 50, 53; Eph 6:24; 2Ti 1:10.

The expressions “immortal” or “immortality” do not occur in the Hebrew Scriptures, which do show, however, that Jehovah God, as the Source of all life, is not subject to death, hence, is immortal. (Ps 36:7, 9; 90:1, 2; Hab 1:12) This fact is also emphatically stated by the Christian apostle Paul in referring to God as “the King of eternity, incorruptible.”​—1Ti 1:17.

As the article SOUL shows, the Hebrew Scriptures also make plain that man is not inherently immortal. References to the human soul (Heb., neʹphesh) as dying, heading for the grave, and being destroyed are numerous. (Ge 17:14; Jos 10:32; Job 33:22; Ps 22:29; 78:50; Eze 18:4, 20) The Christian Greek Scriptures, of course, are in harmony and likewise contain references to the death of the soul (Gr., psy·kheʹ). (Mt 26:38; Mr 3:4; Ac 3:23; Jas 5:20; Re 8:9; 16:3) Therefore the Christian Greek Scriptures do not dispute or alter the inspired teaching of the Hebrew Scriptures that man, the human soul, is mortal, subject to death. The Christian Greek Scriptures, however, do contain the revelation of God’s purpose to grant immortality to certain of his servants.

How can Jesus be “the one alone having immortality”?

The first one described in the Bible as rewarded with the gift of immortality is Jesus Christ. That he did not possess immortality before his resurrection by God is seen from the inspired apostle’s words at Romans 6:9: “Christ, now that he has been raised up from the dead, dies no more; death is master over him no more.” (Compare Re 1:17, 18.) For this reason, when describing him as “the King of those who rule as kings and Lord of those who rule as lords,” 1 Timothy 6:15, 16 shows that Jesus is distinct from all such other kings and lords in that he is “the one alone having immortality.” The other kings and lords, because of being mortal, die, even as did also the high priests of Israel. The glorified Jesus, God’s appointed High Priest after the order of Melchizedek, however, has “an indestructible life.”​—Heb 7:15-17, 23-25.

The word “indestructible” here translates the Greek term a·ka·taʹly·tos, meaning, literally, “indissoluble.” (Heb 7:16, ftn) The word is a compound of the negative prefix a joined to other words relating to a “loosening down,” as in Jesus’ statement regarding the loosening down or throwing down of the stones of the temple at Jerusalem (Mt 24:1, 2), as well as in Paul’s reference to the loosening down of the earthly “tent” of Christians, that is, the dissolving of their earthly life in human bodies. (2Co 5:1) Thus, the immortal life granted Jesus upon his resurrection is not merely endless but is beyond deterioration or dissolution and is beyond destruction.

Kingdom Heirs Granted Immortality. For the anointed Christians called to reign with Christ  in the heavens (1Pe 1:3, 4), the promise is that they share with Christ in the likeness of his resurrection. (Ro 6:5) Thus, as in the case of their Lord and Head, the anointed members of the Christian congregation who die faithful receive a resurrection to immortal spirit life, so that “this which is mortal puts on immortality.” (1Co 15:50-54) As with Jesus, immortality in their case does not mean simply everlasting life, or mere freedom from death. That they, too, are granted “the power of an indestructible life” as fellow heirs with Christ is seen from the apostle Paul’s association of incorruptibility with the immortality they attain. (1Co 15:42-49) Over them “the second death has no authority.”​—Re 20:6; see INCORRUPTION.

This grant of immortality to the Kingdom heirs is all the more remarkable, in view of the fact that even God’s angels are shown to be mortal, despite their possessing spirit bodies, not carnal ones. Angelic mortality is evident in view of the judgment of death entered against the spirit son who became God’s Adversary, or Satan, and also against those other angels who followed that satanic course and “did not keep their original position but forsook their own proper dwelling place.” (Jude 6; Mt 25:41; Re 20:10, 14) So the grant of “indestructible life” (Heb 7:16) or “indissoluble life” to those Christians who gain the privilege of reigning with God’s Son in the heavenly Kingdom marvelously demonstrates God’s confidence in them.​—See HEAVEN (The way to heavenly life); LIFE.