2) Natural Human Mortality
The label "conditional immortality" emphasizes the theological anthropology inherent in an annihilationist view of the final judgment of the wicked. Specifically, it refers to the idea that the Bible teaches that human beings were created mortal and remain mortal unless they partake of the Tree of Life (whether it is a literal, sacramental tree related to God's bestowal of eschatological covenant blessing, or whether that tree is simply a literary symbol for the same).
Before the Fall, at least on a Vosian reading of the covenantal arrangement in the garden of Eden, Adam was put on probation and upon passing the probationary test of obedience would have been granted to eat of the Tree of Life and thus advance human nature from natural weakness and corruptability/mortality to glorious, Holy Spirit-infused immortality. In 1 Cor. 15:42-49, Paul coordinates fallen "natural" man with even unfallen "natural man" (see v.44b as the transition point in the argument) when contrasting both with the body that is raised with glory and power in the resurrection, reflecting the "image of the heavenly" [i.e. the risen Christ]. In the next paragraph (vv. 50-57), this contrast is further developed in explicit terms of "perishable" vs. "imperishable," "mortal" and "immortal[ity]", and Paul refers to Isaianic prophecy of the swallowing up of Death.
When Adam and Eve fell, they were driven from the garden and prevented from partaking of the Tree of Life, lest man "stretch out his hand, and take also from the tree of life, and eat, and live forever" (Gen. 3:22). God told Adam, "By the sweat of your face You will eat bread, till you return to the ground because from it you were taken; for you are dust, and to dust you shall return" (v. 19). The Bible seems to describe an at least partially active role of God in the final judgment of the wicked at the resurrection day, but from one perspective, the original plunging of humanity into death was more of a failure to attain an offered blessing of escape from death than it was "death as an utterly new potentiality" imposed upon man's reality. Man would remain mortal, until life and immortality would be "brought to light" by the grace of God (2 Tim. 1:10). It is only by virtue of redemption in Jesus Christ, because of his substitutionary death and resurrection, that believers in him may again partake of the Tree of Life (Rev. 22:14).
By faith alone in Christ alone--by "eating" the "bread of heaven"--human beings may experience everlasting life (Jn. 6:51, 58). There is no biblical evidence that God will grant a "different kind" of "immortality" to the wicked for the purpose of causing them to "live" in Gehenna and undergo eternal physical torment; salvation is always "out of death into life" (Jn. 5:24). Outside of the resurrection life granted in Christ, the wicked will rise only for a formal judgment/reckoning before the throne of God, but they will ultimately be killed and cease to live, forever.
There is likewise no biblical evidence that human beings in general--much less the wicked--are created with inherently immortal souls (a rather more Greek than traditionally Jewish idea). God alone possesses immortality (1 Tim. 6:16) and shares it by the grace given in Christ, of which the finally impenitent will have no part, eternally--they will perish. If humans have souls that are distinct substances from the body, constitutive of complete human nature together with the body, the Bible regards that part of man as just as inherently mortal as the body, apart from the eschatological covenant blessing of God whether by the Tree of Life in the garden (but that ship has sailed) or by means of the Tree of Calvary.
"This mortal must put on immortality" (1 Cor. 15:53b, my emphasis).
