Whether one interprets John 8:58 in light of Exodus 3:14 (I do not), the Unitarian still has quite a big problem. Here's why. The association with the Feast of Tabernacles and the Feast of Dedication (also known as “Second Tabernacles”) provides us with some context to Jesus’ statements. Jesus’ run in with the Jews unfolds at the heel of the Feast of Tabernacles (John 7:1-10:21); the very occasion Ps. 95 and Deut. 32 would have been sung by the congregations of Israel (Deut. 31:21-32:43),
Behold, behold that I am (ἐγώ εἰμι),
And there is no god besides Me.
I kill, and I will make alive;
I will smite, and I will heal;
And there is no one who shall deliver out of My hand (καὶ οὐκ ἔστιν ὃς ἐξελεῖται ἐκ τῶν χειρῶν μου). (Deut. 32:39 LXX)
The verbal similarities between Jesus statements in John 8:58 and John 10:27-36 is intended to mimic songs of praise to YHWH that would have been sang on the very evening of.
Notice that in John 8:58, Jesus does not say, “Before Abraham came into existence, I came into existence.” Rather, Jesus expresses an existence of a different order than that of Abraham by contrasting between Abraham, who “came into being” (genesthai), and He, who simply is (ego eimi).
By itself, ego eimi does not imply eternal pre-existence; however, when placed alongside genesthai and referring to a time anterior to that indicated by genesthai (“came into being”), ego eimi or its related forms (because it denotes simple existence and is a durative form of the verb “to be”) stands in sharp contrast to the aorist genesthai which speaks of “coming into being.” It is this sharp contrast between being and becoming which makes it clear that in a text like John 8:58 that ego eimi implies eternality, not merely temporal priority. The statement echoes the classic affirmation of the eternal being of God in the OT: “Before the mountains came into being (genethenai) and the earth and world were formed, even from age to age, You are (su ei, second-person equivalent of ego eimi),” Psalm 90:2 (see LXX).
With this in purview, we can see the import of Jesus’ words quite clearly. Keep in mind that just a few verses prior (John 8:40), Jesus states that, unlike the Jews of Jesus’ day,
Abraham did not try to kill Him,
But now you are trying to kill me, a man who has told you the truth I heard from God; this Abraham did not do (John 8:40)
But there remains a question: At what point in time would Abraham even had seen Jesus to have the opportunity to do so?
I think this is best answered if we read John 8 through the lens of the prologue, John 1:1. Notice in John 1:1, Jesus is identified as “the Word,” and that Word existed in a “face-to-face” relationship with God (John 1:1b) — πρὸς τὸν θεὸν. This πρὸς τὸν θεὸν (John 1:1b) language or some slight variation thereof, is used throughout the Testaments — some (17) occurrences of the phrase found within the NT, according to NA28 (Jn. 1:1, 2, 13:3; 1 Jn 3:21; Rev. 12:5, 13:6; Acts 4:24, 12:5, 24:16; Romans 5:1, 10:1, 15:30; 2 Cor. 3:4, 13:7; Philippians 4:6; 1 Thess. 1:8, 9) — and another 20+ examples in the Genesis and Exodus accounts of the Greek OT/LXX (Gen. 17:18, 18:27, 18:31, 20:17, 24:49, 24:54, 24:46; Exodus 2:23, 3:11, 3:13, 8:25, 8:26, 9:29, 10:18, 18:19, 19:8, 19:21, 19:23, 19:24, 24:1, 24:2, 32:30). In each of these occurrences (aside from the occurrences where the neuter article τὰ is present), they explicitly refer to distinct individuals in some form of communication with one another. And it will show back up here in a moment.
John 1:18 closes the prologue with an admittedly curious phrase, “No one has ever seen God.” Yet, throughout Scripture men are said to have seen God, even “face-to-face.” For instance, Genesis 15:1 states,
the Word of the LORD came to Abram in a vision (Genesis 15:1 LXX)
And notice the language here is similar to what’s found in John 1:1b — ῥῆμα Κυρίου πρὸς Ἀβρὰμ. The only difference is that instead of “the Word” being “with God,” it is “the Word of the LORD” being “with (or ‘came to’) Abram.”
And in what manner does Abram respond, but with the following word:
Abram said, ‘O Lord God’ (Genesis 15:2)
Hence, Abram saw the Word, and addressed Him as none other than YHWH. The second clause of John 1:18 — “He who is at the Father’s side, He has made Him known” — provides valuable insight. Just as the Word (who came in a “vision” and “appeared” to Samuel) made YHWH known,
Now Samuel did not yet know the Lord, nor had the Word of the Lord yet been revealed to him. So the Lord called Samuel again for the third time. And he arose and went to Eli and said, ‘Here I am, for you called me.’ Then Eli discerned that the Lord was calling the boy… And the Lord appeared again at Shiloh, because the Lord revealed Himself to Samuel at Shiloh by the Word of the Lord. (1 Samuel 3:7-9, 21)
So too has Jesus made known (or exegeted) the Father (John 1:18). Hence, Abram saw God through the agency of the Word. Everyone who has ever seen YHWH, has in fact seen the “one and only God,” as He is the one who exposits God to men (ἐξηγέομαι) — He who is “the image of the invisible God.” Because Jesus (as the Word) is, as to His nature, God; He is then thereby qualified to perfectly reveal/exegete the Father (cf. John 12:45, 14:9), as He is the exact reflection of God's very being.
This flies in the face of all brands of Unitarianism. It decimates Socinianism (or “Biblical Unitarianism”) and all forms of Modalism (or “Sabellianism”), as the language of John 1:1b (and all other like occurrences as cited above) refutes the notion that “the Word” is not a person. Likewise, John 1:18 draws a correlation between Jesus and that mysterious OT figure known as “the Word,” who revealed YHWH to men (including Samuel, and Abraham). And for the Jehovah’s Witness, what sort of god does Abram identify “the Word” as? Who alone, but “O Lord God”? This is the one who “became flesh and tabernacled among us,” John 1:14 (cf. Exodus 36:8–39:43).
And he made the middle bar to mortise in the midst of the boards from end to end—of the tree which our father Abraham planted in Beira of Sheba, praying there in the Name of the Word of the Lord, the everlasting God. (Pal. Targum, Exodus 36:33)