Structural ambiguity exists “when a given word or phrase…can be taken as modifying any one of two (or more) different constituents” (Radford 66). It is not uncommon for languages to exhibit structural ambiguity. For example, in the sentence “The police will shoot terrorists with rifles” (Radford 106), the prepositional phrase with rifles could be understood in two different ways:

1. modifying the verb shoot (in which case the police are the ones with rifles)
2. modifying the noun terrorists (in which case the terrorists are the ones with rifles)

In the Gospel of John there is an interesting case of structural ambiguity involving the prepositional phrase ἐν αὐτῷ (“in him”). Here is the clause as it is in Greek: ἵνα πᾶς ὁ πιστεύων ἐν αὐτῷ ἔχῃ ζωὴν αἰώνιον (John 3:15). Does “in him” modify the preceding participle πιστεύων? Or does it modify some part of the following phrase ἔχῃ ζωὴν αἰώνιον?

"Believe in him" or "eternal life in him"?

Greek syntax allows for both options. And both options are represented in various English translations:

NIV (1984) “that everyone who believes in him may have eternal life”

NIV (2011) “that everyone who believes may have eternal life in him“

ESV “that whoever believes in him may have eternal life”

NCV “So that everyone who believes can have eternal life in him“

NLT “so that everyone who believes in him will have eternal life”

NLT (margin) “everyone who believes will have eternal life in him“

NASB “so that whoever believes will in Him have eternal life”

NASB (margin) “believes in Him will have eternal life”

KJV “That whosoever believeth in him should not perish, but have eternal life”

So which is correct? Although both are true doctrinally, a closer look at how John uses believing-in-him language elsewhere tips the scales in one direction.

A search on my Logos Bible Software turned up the following verses for the lemma πιστεύω within 3 words of the prepositional phrase ἐν αὐτῷ in the Gospel of John. Notice the first hit is the verse under consideration, while the second hit is actually a false hit because the prepositional phrase is actually part of the previous clause and does not modify πιστεύω.

John 3:15 ἵνα πᾶς ὁ πιστεύων ἐν αὐτῷ ἔχῃ ζωὴν αἰώνιον.

1 John 5:10 ὁ πιστεύων εἰς τὸν υἱὸν τοῦ θεοῦ ἔχει τὴν μαρτυρίαν ἐν αὐτῷ, ὁ μὴ πιστεύων τῷ θεῷ ψεύστην πεποίηκεν αὐτόν, ὅτι οὐ πεπίστευκεν εἰς τὴν μαρτυρίαν ἣν μεμαρτύρηκεν ὁ θεὸς περὶ τοῦ υἱοῦ αὐτοῦ.

On the other hand, ἐν αὐτῷ is not the only prepositional phrase available to communicate the idea. A writer could also use εἰς αὐτὸν to communicate the same idea. As it turns out,  a search for the lemma πιστεύω within 3 words of the prepositional phrase εἰς αὐτὸν in the Gospel of John turned up many more results.

John 2:11 Ταύτην ἐποίησεν ἀρχὴν τῶν σημείων ὁ Ἰησοῦς ἐν Κανὰ τῆς Γαλιλαίας καὶ ἐφανέρωσεν τὴν δόξαν αὐτοῦ, καὶ ἐπίστευσαν εἰς αὐτὸν οἱ μαθηταὶ αὐτοῦ.

John 3:16 οὕτως γὰρ ἠγάπησεν ὁ θεὸς τὸν κόσμον, ὥστε τὸν υἱὸν τὸν μονογενῆ ἔδωκεν, ἵνα πᾶς ὁ πιστεύων εἰς αὐτὸν μὴ ἀπόληται ἀλλʼ ἔχῃ ζωὴν αἰώνιον.

John 3:18 ὁ πιστεύων εἰς αὐτὸν οὐ κρίνεται· ὁ δὲ μὴ πιστεύων ἤδη κέκριται, ὅτι μὴ πεπίστευκεν εἰς τὸ ὄνομα τοῦ μονογενοῦς υἱοῦ τοῦ θεοῦ.

John 4:39 Ἐκ δὲ τῆς πόλεως ἐκείνης πολλοὶ ἐπίστευσαν εἰς αὐτὸν τῶν Σαμαριτῶν διὰ τὸν λόγον τῆς γυναικὸς μαρτυρούσης ὅτι εἶπέν μοι πάντα ἃ ἐποίησα.

John 6:40 τοῦτο γάρ ἐστιν τὸ θέλημα τοῦ πατρός μου, ἵνα πᾶς ὁ θεωρῶν τὸν υἱὸν καὶ πιστεύων εἰς αὐτὸν ἔχῃ ζωὴν αἰώνιον, καὶ ἀναστήσω αὐτὸν ἐγὼ [ἐν] τῇ ἐσχάτῃ ἡμέρᾳ.

John 7:5 οὐδὲ γὰρ οἱ ἀδελφοὶ αὐτοῦ ἐπίστευον εἰς αὐτόν.

John 7:31 Ἐκ τοῦ ὄχλου δὲ πολλοὶ ἐπίστευσαν εἰς αὐτὸν καὶ ἔλεγον· ὁ χριστὸς ὅταν ἔλθῃ μὴ πλείονα σημεῖα ποιήσει ὧν οὗτος ἐποίησεν;

John 7:39 τοῦτο δὲ εἶπεν περὶ τοῦ πνεύματος ὃ ἔμελλον λαμβάνειν οἱ πιστεύσαντες εἰς αὐτόν· οὔπω γὰρ ἦν πνεῦμα, ὅτι Ἰησοῦς οὐδέπω ἐδοξάσθη.

John 7:48 μή τις ἐκ τῶν ἀρχόντων ἐπίστευσεν εἰς αὐτὸν ἢ ἐκ τῶν Φαρισαίων;

John 8:30 Ταῦτα αὐτοῦ λαλοῦντος πολλοὶ ἐπίστευσαν εἰς αὐτόν.

John 9:36 ἀπεκρίθη ἐκεῖνος καὶ εἶπεν· καὶ τίς ἐστιν, κύριε, ἵνα πιστεύσω εἰς αὐτόν;

John 10:42 καὶ πολλοὶ ἐπίστευσαν εἰς αὐτὸν ἐκεῖ.

John 11:45 Πολλοὶ οὖν ἐκ τῶν Ἰουδαίων οἱ ἐλθόντες πρὸς τὴν Μαριὰμ καὶ θεασάμενοι ἃ ἐποίησεν ἐπίστευσαν εἰς αὐτόν·

John 11:48 ἐὰν ἀφῶμεν αὐτὸν οὕτως, πάντες πιστεύσουσιν εἰς αὐτόν, καὶ ἐλεύσονται οἱ Ῥωμαῖοι καὶ ἀροῦσιν ἡμῶν καὶ τὸν τόπον καὶ τὸ ἔθνος.

John 12:37 Τοσαῦτα δὲ αὐτοῦ σημεῖα πεποιηκότος ἔμπροσθεν αὐτῶν οὐκ ἐπίστευον εἰς αὐτόν,

John 12:42 ὅμως μέντοι καὶ ἐκ τῶν ἀρχόντων πολλοὶ ἐπίστευσαν εἰς αὐτόν, ἀλλὰ διὰ τοὺς Φαρισαίους οὐχ ὡμολόγουν ἵνα μὴ ἀποσυνάγωγοι γένωνται·

What this suggests is that when John wants to talk about ‘believing in him’ he uses the prepositional phrase εἰς αὐτὸν and not ἐν αὐτῷ. So when we come across the phrase ἐν αὐτῷ in John 3:15 it almost certainly is not modifying πιστεύω but rather some part of the following phrase ἔχῃ ζωὴν αἰώνιον. That is, the clause should be translated as “everyone who believes may have eternal life in him.”