“Might” have eternal life?

In our weekly Greek class, we’ve been learning the basic mood known as the “indicative.” It is the most common verbal mood and deals with what is real, taking the form of either a statement or a question. Other Greek moods include the subjunctive (a probability), imperative (a command), and optative (a wish).
In a sense, the subjunctive mood is one step removed from reality, and often deals with what “might” or “could” to be. Sometimes, it deals with probability. But not always.
We haven’t gotten to the subjunctive mood yet in Mounce’s Basics of Biblical Greek yet (see ch. 31), but a student raised an interesting question last night about might/should/will in John 3:16 and our promise of everlasting life.
Greek: “μὴ ἀπόληται ἀλλ ̓ ἔχῃ ζωὴν αἰώνιον.”
NASB: “shall not perish, but have eternal life.”
ESV: “should not perish, but have eternal life.”
Is there some uncertainty here? No. Apparently, the subjunctive mood was used by John out of grammatical necessity. The subjunctive is used simply because it follows the conjunction ἵνα, “in order that”. Mounce explains this use on p. 293:

31.13.  1. ἵνα and the subjunctive. ἵνα is almost always followed by the subjunctive and can indicate purpose.

Here’s a more in-depth explanation from Wallace’s Greek Grammar Beyond the Basics, p. 474:

3) Subjunctive mood used in a Purpose-Result Ἳνα Clause
Not only is ἵνα used for result in the NT, but also for purpose-result. That is, it indicates both the intention and its sure accomplishment. BAGD point out in this connection: “In many cases purpose and result cannot be clearly differentiated, and hence ἵνα is used for the result which follows according to the purpose of the subj[ect] or of God. As in Jewish and pagan thought, purpose and result are identical in declarations of the divine will.” Likewise, Moule points out that “the Semitic mind was notoriously unwilling to draw a sharp dividing-line between purpose and consequence.” In other words, the NT writers employ the language to reflect their theology: what God purposes is what happens and, consequently, ἵνα is used to express both the divine purpose and the result.
This probably does not represent a change in syntax from classical to Koine, but a change in subject matter. It is, of course, possible to treat each of these examples as simply purpose ἵνα clauses in which there is evidently no doubt about the accomplishment from the speaker’s viewpoint. Hence, in order that is an acceptable gloss.
Jn. 3:16 τὸν υἱὸν τὸν μονογενῆ ἔδωκεν, ἵνα πᾶς ὁ πιστεύων εἰς αὐτὸν μὴ ἀπόληται ἀλλ ̓ ἔχῃ ζωὴν αἰώνιον
He gave his only Son, in order that everyone who believes in him should not perish but should have eternal life.
The fact that the subjunctive is all but required after ἴνα does not, of course, argue for uncertainty as to the fate of the believer. This fact is obvious, not from this text, but from the use of of οὐ μή in John 10:28 and 11:26, as well as the general theological contours of the gospel of John.

So, there you have it. By using ἵνα + the subjunctive mood, the Apostle John says that both the original purpose and the accomplished result of our faith in Christ is rescue from death and the gift of life! Eternal life is not just a possibility. It is an absolute certainty for those who believe.