Here, John is making the most categorical and concise statement found anywhere in scripture regarding the deity of Jesus - He is GOD. If you need a more detailed explanation on this I would recommend you read the the answers posted on the following thread.
"Why John 1:1 in (DRB)(Douay-Rheims Bible) is not literal translation from the Latin Vulgate?"
The meaning is revealed by the unusual construction of και θεος ην ο λογος. Here is what I mean.
The following are arguments by a professor of NT Greek given to me years ago whose name I no longer remember. Over the years, I have rewritten most of it to make the arguments more concise and easier to understand, but his arguments remain the same.
First, Koine Greek normally drops the article in a prepositional phrase. The absence of the article in a prepositional phrase is normal and doesn't mean anything. It is the INCLUSION of the article in a prepositional phrase as we see in this verse that is unusual and thus means something.
The prepositional phrase "εν αρχη" for example, does not contain an article, but is still properly translated "in the beginning." The prepositional phrase "προς τον θεον," however, does include the article (τον). Since it would have been grammatically proper not to include it, then the INCLSION of the article here means something. In general, the inclusion of an article in Greek when it is not expected means the writer is being specific. There are three things this could mean, depending on the construction:
a. It could mean that the Word was a LESSER god than the Father who is the τον θεον (the God) in the previous clause.
b. Or, it could mean that the Word was the Father.
c. Or, it could mean that the Word was fully God, but was NOT the same person as Father.
So, the question is this, how do we determine which is meant?
If John had written the clause: και ο λογος ην θεος, it would mean that "the Word was 'A' god." That is, the Word was a LESSER god than the Father. The reason is that since λογος is the subject and appears first, there is no grammatical reason to leave the article off of θεος, thus the absence of the article means something (since even if it was given the article, it would STILL be the predicate). Therefore, the absence of the article would mean "A" god. In other words, since the inclusion of the article would not change the grammatical function of θεος, the exclusion of the article must therefore change the MEANING of θεος.
The absence of the article in a position where the inclusion of the article would NOT change the word's grammatical function would tell us there is a difference in specificity: the λογος is not the same individual as the Father.
Further, if it does not have an article, the position of θεος at the end of the sentence would tell us there is a difference in emphasis (θεος is being “de-emphasized”): In such a construction, λογος would then be less of a god than the Father. Thus, "και ο λογος ην θεος" could ONLY mean "the Word was a god." BUT, John did NOT use this construction.
If John had written the clause: και ο λογος ην ο θεος, it would mean that "the Word was THE God." In other words, the Word was exactly the same person as the Father. This would mean only one person is being represented throughout the text, not two. The Father and the Son would then be nothing more than manifestations of the same person. They would not be separate individuals. It would mean here is one personality who simply appears at times in different forms. This would then lend support to the modalist argument. The inclusion of the article with θεος would make it specific and make θεος the subject: In such a construction, the λογος would be exactly the same individual as the Father (the exact same θεος just mentioned in the previous clause). Since both nouns have the article, θεος is grammatically locked into occurring after λογος. If it were moved in front of λογος, it would change its grammatical function, and become the subject. Thus, in this construction, the position of θεος would not mean anything. It MUST appear there. Thus, the clause "και ο λογος ην ο θεος" can only mean "Jesus was THE God (the exact same individual as the Father)." BUT, John did NOT use this construction.
By writing it as και θεος ην ο λογος, John does two critical and deliberate things. First, he leaves the article off of θεος, thus indicating that Word is NOT the same individual as the father. Second, he places θεος to the front of the clause which places extra emphasis on the Word. By doing that, John makes it clear by the increase in emphasis, that the absence of the article does NOT mean "lesser." Since the absence of the article does not mean "a lesser god," it leaves us with only one choice as to what is meant: The Word is not exactly the same individual as the "τον θεον" of the second clause, but every bit as much GOD as the "τον θεον" of the second clause. Thus, the absence of the article tells us that the θεος of the third clause is NOT the same individual as the τον θεον of the second clause. The position tells us that the absence of the article does NOT mean "lesser." By placing θεος in a position of emphasis, John is doing the equivalent of bolding it, underlining it, and adding an exclamation point: "The Word was God!"
Now we see why John included the article in the prepositional phrase "προς τον θεον." He was being very specific. The Word is WITH a SPECIFIC being called "The God" (τον θεον). In the next clause, John then lets us know that the Word was completely EQUAL with "The God" in divinity, but through the careful use of the articles John has clued us in that the Word is not the same individual as The God of the previous clause.
One of the objections raised to the divinity of Jesus is that λογος means “the mind, wisdom, intelligence, or plan of God” and nothing more. It is argued that λογος is not an individual but simply a way of describing the “mind” or “wisdom” of God (this was a common philosophy of the Gnostics). Thus, the λογος was not an individual, but simply the wisdom of God, not a “God” made flesh, but the wisdom of God or the mind of God made flesh. That means He did not exist prior to His birth (as God). Prior to his physical birth, this theory suggests that he was merely an idea, or a plan in the MIND of God and that IDEA became a man.
John's construction of this verse is so carefully crafted that it is often called the most concise theological statement ever made. In this one short verse, the Holy Spirit wrote through John a sentence that took me all of this time and space to explain. The Holy Spirit's deliberate use of grammar leaves us only ONE possible conclusion: Jesus is completely and totally God in every way that the Father is God, but Jesus is NOT the same individual as the Father.
If you would like to further investigate the arguments I have presented here, you can access the works of the renowned Dr. Jan Van der Watt on the Gospel long.