A case for high Christology (that Jesus was eternally divine) need not rest on John's gospel exclusively, or even primarily . I take it as a given that John’s gospel does represent Jesus as a pre-existent divine figure, since in John's gospel he claims that he was the "I AM" before Abraham was born (John 8:58) and that He existed with the Father before creation (John 1:1-13). Furthermore, He is practically worshiped by Thomas as God rather than as some angel or exalted divine being, and Jesus never rebuked him (John 20:28).
I would place a case for early, high christology all the way back in Philippians 2:5-11, which was written around 54 AD, very likely before any of the gospels. It is likely also a hymn or a creed, taking it further back. That Paul describes Jesus as a divine figure in this passage isn't disputed by even some of the most prominent skeptics. If you read verse 10-11 of chapter 2, it reads: "that at the name of Jesus every knew should bow, in heaven and on earth and under the earth, and every tongue confess that Jesus Christ is Lord, to the glory of God the father." This part is a quote from a text of Isaiah (Is. 45:23) ascribed directly to YHWY.
That Paul with almost complete consistency referred to the Father as God and to Jesus as Lord is not something I'd dispute or be bothered about. I simply don't think it means that Jesus wasn't God, but rather that Paul was sensitive to the distinct roles of the Son and the Father. Similarly, exaltation verses like Acts 2:36 and Romans 1:4 are specific in saying that the Son's role as Lord of His church and Mediator of His people was publicly granted to Him on behalf of the fulfillment of His divine task in the state of his humanity. This is, in fact, the teaching of the gospel with the highest Christology (John) and is the most consistent view.
I'm also quite open to conceding that "Son of God" could be a title given to an angel or a king in the Jewish worldview. I, therefore, don't claim any argument on that although I do think the title, when applied to Jesus, is meant in a clearly unique way. I prefer to look at the Son of Man sayings in which Jesus refers to Himself, once again, as a clearly divine figure. See Mark 14:62, Psalm 110:1 and Da 7:13. Jesus calls himself Son of Man in this sense in EVERY early gospel tradition: the four strands of material unique to each gospel, and the Q source . Furthermore, it is not a term used by the early church fathers in referring to Jesus, meaning it meets not only the historical criteria for multiple, independent attestation but also the criteria for dissimilarity.
Lastly, if you take the consensus view that Mark was written as the first gospel, you have Jesus calming the storm by his own command (Mark 4:31), transfiguring as the fulfillment of OT law and prophecy (Mark 9:2-12), healing the sick and forgiving sin upon his own authority (Mark 2:5), and being condemned for blasphemy (Mark 14:63-64). Clearly, his disciples and enemies made the leap that many skeptics find difficult to make.
The resurrection certainly sealed the deal, but it wasn't as if the belief in Jesus eternal divinity was made up, based upon that miracle. The resurrection was the confirmation of claims that Jesus had made subtly but, in my view, undeniably throughout his ministry.
 Christology refers to the study of Jesus' person and nature. Per my understanding, a distinction ican be drawn between the view of a pre-existent divine Christ (high Christology) and an exaltation theology (or, as some say, low Christology), wherein Jesus only gained the status of divinity upon his resurrection (or another time during his ministry). There are different ways of using these terms, but this is how I use them in this article, in which I argue for a high Christology based on certain historical facts about Jesus. The terminology and the arguments can get a bit technical, but I hope to make a minimal case that viewing Jesus as a pre-existent divine figure isn't a massive leap when one considers all of the relevant data. If you are interested in this argument further, I'd recommend Ehrman's moderately skeptical book, "How Jesus Became God" and, from a conservative perspective: Baucham's "Jesus and the Eyewitnesses," or Brant Pitre's "The Case for Jesus."
 Gary Habermas, “Recent Perspectives on the Reliability of the Gospels” (Originally published in the Christian Research Journal / vol. 28, no. 1, 2005) at http://www.garyhabermas.com/articles/crj_recentperspectives/crj_recentperspectives.htm#_edn23.