John 1:1

In the beginning was the Word, and the Word was with God, and what God was, the Word was.

Ἐν ἀρχῇ ἦν ὁ λόγος, καὶ ὁ λόγος ἦν πρὸς τὸν θεόν, καὶ θεὸς ἦν ὁ λόγος.

Greek

Compare with the ASV

In the beginning was the Word, and the Word was with God, and the Word was God.

What we did

We rendered the last clause "what God was, the Word was."

Why

The article
In the second clause, "God" has the article: πρὸς τὸν θεόν, "with the God." In the third clause, θεός has no article.
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English has no way to show that difference unless the wording changes, and "the Word was God" reads the same as if the article were there.
The word order
In the third clause, θεός comes first, before the verb, and ὁ λόγος (with the article) is the subject.
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A noun without the article placed before the verb like this describes what the subject is like. Grammarians call this a qualitative use (P. B. Harner, Journal of Biblical Literature 92, 1973).
The wording
"What God was, the Word was" carries the description without adding words that are not in the Greek.
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It is the rendering of the New English Bible and the Revised English Bible.

Other renderings

"the Word was God" (ASV, KJV, ESV); "the Word was divine" (Moffatt, Goodspeed); "the Word was a god" (some other versions).