Let’s look at the relevant verses of John 8:
56 Your father Abraham was overjoyed to see my day, and he saw it and was glad.” 57 Then the Judeans replied, “You are not yet fifty years old! Have you seen Abraham?” 58 Jesus said to them, “I tell you the solemn truth, before Abraham was, I am!” (John 8:56-58)
The critical phrase is the last one: in Greek, prin abraam genesthai egô eimi and, even more specifically, the phrase prin abraam genesthai.
Now, genesthai is the Second Aorist – Middle Deponent – Infinitive form of gi[g]nomai, which has a very broad spectrum of meanings (see Liddel-Scott-Jones Greek Lexicon), the most fundamental of which is “to become” and, only derivatively, “to be [born]” (absolute sense). Normally, in the sense of “to become”, genesthai is followed by a predicate, which is apparently not the case in John 8:58, so, the Greek phrase prin abraam genesthai is usually understood as though genesthai was actually used in the absolute sense, and consequently translated with something like “before Abraham was [born]”.
The English Unitarian Thomas Belsham (1750 – 1829), to interpret John 8:58, considered, among other arguments, that of double ellipsis (see A Calm Inquiry Into the Scripture Doctrine Concerning the Person of Christ, § 3, pp. 53-55).
The argument runs, more or less, like this. If, instead of interpreting genesthai as absolute (“to be [born]”), we suppose that genesthai means “to become” (in an elliptical sense to be determined), the key phrase at John 8:58 becomes:
[Lit. Eng.] before Abraham become [ellipsis], I am [ellipsis]
Neither “become”, nor “I am” are, reasonably, used in an absolute sense. So, there may be an ellipsis associated with each verb. Unpacking the [double ellipsis], we may have:
“Before Abraham become [father of a multitude (viz. of nations) – Gen 17:5] I am [the Messiah]”
Less obscurely, what Jesus is saying to “the Jews” may be something like this:
“And verily I say, that the time for the accomplishment of what he foresaw is not yet arrived: for before Abram shall be Abraham, i. e. become the father of many nations, according to the import of his name, I am the Christ your Messiah.” (Interpretation and paraphrase of John 8:58 provided at the Theological Repository vol. iv. p. 351, as quoted by Thomas Belsham in A Calm Inquiry Into the Scripture Doctrine Concerning the Person of Christ, p. 54)
Belsham notes that this interpretation of John 8:58 was first proposed by the Polish Socinians (presumably, the Polish Brethren, a non-trinitarian Protestant church that existed in Poland from 1565 to 1658).
