What is the biblical basis for the immortality of the soul?

First, we must prove that there is a soul within man which can exist outside the body, i.e. disembodied, for there are some who deny that man has a soul, but instead, they assert that he is a soul (which objection is addressed here).

In 2 Cor. 12:2-4, the apostle Paul wrote,

² I know a man in Christ fourteen years ago (I don't know whether in the body or outside the body; God knows), such a man was caught away unto the third heaven. ³ And I know such a man (I don't know whether in the body or outside the body; God knows) that he was caught away into Paradise, and he heard unspeakable words which are not lawful for a man to utter.

² οἶδα ἄνθρωπον ἐν Χριστῷ πρὸ ἐτῶν δεκατεσσάρων εἴτε ἐν σώματι οὐκ οἶδα εἴτε ἐκτὸς τοῦ σώματος οὐκ οἶδα ὁ θεὸς οἶδεν ἁρπαγέντα τὸν τοιοῦτον ἕως τρίτου οὐρανοῦ ³ καὶ οἶδα τὸν τοιοῦτον ἄνθρωπον εἴτε ἐν σώματι εἴτε ἐκτὸς τοῦ σώματος οὐκ οἶδα ὁ θεὸς οἶδεν ⁴ ὅτι ἡρπάγη εἰς τὸν παράδεισον καὶ ἤκουσεν ἄῤῥητα ῥήματα ἃ οὐκ ἐξὸν ἀνθρώπῳ λαλῆσαι

Henry Alford wrote,

The adoption of the third person is remarkable: it being evident from 2 Corinthians 12:7 that he himself is meant. It is plain that a contrast is intended between the rapt and glorified person of 2 Corinthians 12:2; 2 Corinthians 12:4,—and himself, the weak and afflicted and almost despairing subject of the σκόλοψ τῇ σαρκί of 2 Corinthians 12:7 ff. Such glory belonged not to him, but the weakness did. Nay, so far was the glory from being his, that he knew not whether he was in or out of the body when it was put upon him: so that the ἐγὼ αὐτός, compounded of the νοῦς and σάρξ (Romans 7:25), clearly was not the subject of it, but as it were another form of his personality, analogous to that which we shall assume when unclothed of the body.

It may be remarked in passing, as has been done by Whitby, that the Apostle here by implication acknowledges the possibility of consciousness and receptivity in a disembodied state.

It's evident from that passage that man can have existence outside the body (ἐκτὸς τοῦ σώματος), and if not as a sentient soul, then what? Indeed, it was as a soul, or if you wish, a spirit (for the two are sometimes treated as synonymous), that Lazarus, the rich man, and Abraham (who had died long before) were all in Hades, sensible and sentient (Luke 16:19-31).

Having thus proved that the personality can exist as a soul outside the body (disembodied), sensible and sentient, we must now prove that the soul is immortal.

Justin Martyr summarizes this in his Dialogue with Trypho the Jew (Ch. VI):

For the truth is so; and you would perceive it from this. The soul assuredly is or has life. If, then, it is life, it would cause something else, and not itself, to live, even as motion would move something else than itself. Now, that the soul lives, no one would deny. But if it lives, it lives not as being life, but as the partaker of life; but that which partakes of anything, is different from that of which it does partake. Now the soul partakes of life, since God wills it to live. Thus, then, it will not even partake [of life] when God does not will it to live. For to live is not its attribute, as it is God's; but as a man does not live always, and the soul is not for ever conjoined with the body, since, whenever this harmony must be broken up, the soul leaves the body, and the man exists no longer; even so, whenever the soul must cease to exist, the spirit of life is removed from it, and there is no more soul, but it goes back to the place from whence it was taken.' _{(Roberts-Donaldson)}

τὸ γὰρ ἀληθὲς οὕτως ἔχει· μάθοις δ' ἂν ἐντεῦθεν. ἡ ψυχὴ ἤτοι ζωή ἐστιν ἢ ζωὴν ἔχει. εἰ μὲν οὖν ζωή ἐστιν, ἄλλο τι ἂν ποιήσειε ζῆν, οὐχ ἑαυτήν, ὡς καὶ κίνησις ἄλλο τι κινήσειε μᾶλλον ἢ ἑαυτήν. ὅτι δὲ ζῇ ψυχή, οὐδεὶς ἀντείποι. εἰ δὲ ζῇ, οὐ ζωὴ οὖσα ζῇ, ἀλλὰ μεταλαμβάνουσα τῆς ζωῆς· ἕτερον δέ τι τὸ μετέχον τινὸς ἐκείνου οὗ μετέχει. ζωῆς δὲ ψυχὴ μετέχει, ἐπεὶ ζῆν αὐτὴν ὁ θεὸς βούλεται. οὕτως ἄρα καὶ οὐ μεθέξει ποτέ, ὅταν αὐτὴν μὴ θέλοι ζῆν. οὐ γὰρ ἴδιον αὐτῆς ἐστι τὸ ζῆν ὡς τοῦ θεοῦ· ἀλλὰ ὥσπερ ἄνθρωπος οὐ διὰ παντός ἐστιν οὐδὲ σύνεστιν ἀεὶ τῇ ψυχῇ τὸ σῶμα, ἀλλ', ὅταν δέῃ λυθῆναι τὴν ἁρμονίαν ταύτην, καταλείπει ἡ ψυχὴ τὸ σῶμα καὶ ὁ ἄνθρωπος οὐκ ἔστιν, οὕτως καί, ὅταν δέῃ τὴν ψυχὴν μηκέτι εἶναι, ἀπέστη ἀπ' αὐτῆς τὸ ζωτικὸν πνεῦμα καὶ οὐκ ἔστιν ἡ ψυχὴ ἔτι, ἀλλὰ καὶ αὐτὴ ὅθεν ἐλήφθη ἐκεῖσε χωρεῖ πάλιν. _{(Patrologiae Cursus Completus. Series Graeca. Vol. 6)}

The apostle Paul wrote that only God has immortality (1 Tim. 6:16); therefore, since all else is created by God, all else is not immortal. But, what about those of us who will one day become immortal in incorruptible, spiritual bodies, as the apostle Paul wrote in 1 Cor. 15:53,

For this corruptible must put on incorruption, and this mortal must put on immortality.

δεῖ γὰρ τὸ φθαρτὸν τοῦτο ἐνδύσασθαι ἀφθαρσίαν καὶ τὸ θνητὸν τοῦτο ἐνδύσασθαι ἀθανασίαν

Is not man to be immortal also? Evidently so; thus, we must distinguish between (1) inherent immortality and (2) granted immortality. God alone has inherent immortality, that is, "life in Himself" (John 5:26). But, He grants or gives immortality to those He wills.

For example, in John 10:28, it is written,

And I give them eternal life, and they shall certainly never perish, neither shall anyone pluck them out of my hand.

κἀγὼ ζωὴν αἰώνιον δίδωμι αὐτοῖς καὶ οὐ μὴ ἀπόλωνται εἰς τὸν αἰῶνα καὶ οὐχ ἁρπάσει τις αὐτὰ ἐκ τῆς χειρός μου

1. There is a soul.
2. The soul is disembodied, sentient, and sensible upon the death of the body.
3. God grants (gives) immortality to the soul as He wills, but the soul is not inherently immortal.
4. God alone is inherently immortal ("has life in Himself").

It should be noted that immortality is also granted to the unrighteous, but for the purpose of eternal torment rather than eternal life (i.e., reward) (cp. Dan. 12:2)).

• John 5:28: "All who are in the graves shall hear his voice...
• John 5:29: And they shall come forth, those who have done good unto the resurrection of life, and those who have done evil, unto the resurrection of damnation.