Someone might argue that, even though a predicate noun often does not have an article when it precedes the verb (as explained in the previous argument), John could have used an article in this case, just to make it more clear that Jesus was God.
One possible answer is that John was avoiding a theological misunderstanding.
One doctrinal error about the Trinity is called “modalism,” the idea that God exists in three different forms, but that there are not three persons in the same Godhead who exist at the same time. This doctrine was later expressed formally by Sabellius in the 3rd century A.D.
Linguist A. Robertson argues that John leaves out the article before "God" in John 1:1 in order to avoid such a misunderstanding about the Trinity, even before it was expressed by Sabellius. He says:
"By exact and careful language John denied Sabellianism by not saying ho theos een ho logos. That would mean that all of God was expressed in ho logos and the terms would be interchangeable, each having the article." [1]
In other words, if John had used the article before "God," it may have suggested that when Jesus became human, all of God became Jesus, and that He became the new expression of God. But it was not like that. When Jesus took on a human nature, God the Father and God the Holy Spirit continued to exist as before.
[1] Robertson, A. Robertson's Word Pictures in Six Volumes (electronic ed.), Logos software.
