Pros ton Theon

Sabellianism or Modalism

καὶ ὁ λόγος ἦν ὁ θεὸς = “and the Word was the God (the Father)"

Arianism (example = Jehovah's Witness)

καὶ ὁ λόγος ἦν θεὸς = “and the Word was a god”

Trinitarianism (orthodox view)

καὶ θεὸς ἦν ὁ λόγος “and the Word was God (Deity)”

Presence and Absence of the Greek definite article

Why is the definite article included in the words "and the Word was with the God" and yet excluded from the words "and the word was God"?  Does the absence of the definite article in the latter words give license to the Arians to add the indefinite article "a"? 

The absence of the definite article in the last clause is detrimental to Modalism.  Had the Apostle John wanted to teach Modalism then he would have said "and the Word was the God."  The absence of the definite article in the last clause is intended to affirm that "the Logos" is deity, is "God ('theos')" by nature, or essentially, and calls attention to the special quality and character of God's being and person. 

"God" in the first clause clearly denotes the Father and should be interpreted as meaning - "and the Word was with the God (Father)."  "God" in the last clause, however, should be thought of as saying - "and the Word was God (Deity)." 

The absence of the definite article does not mean it is indefinite, as the Arians falsely think.  The Greek language did not make use of the indefinite article and so the indefinite article cannot be necessarily implied by the absence of the definite article.  Rather, the design of omitting the definite article was in order to affirm something about the quality or character of the person designated as "the Word." 

Pros ton theon

John's phrase "pros ton theon" ("with the God") conveys the idea of intimate love and fellowship between he who is "the Word" and he who is "the God (Father)."  John purposefully used the preposition "pros" when he writes - “and the Word was with [pros] God.” The preposition pros has various meanings depending on the context and the Greek syntax. However, when applied to individuals pros consistently involves the idea of intimate communion between two persons.

A reliable Internet site gives us these citations and comments (emphasis mine):

"A. T. Robertson correlates John 1:1 and in 2 Corinthians 5:8 "pros ton kurion" (“with the Lord”):

“It is the face-to-face converse with the Lord that Paul has in mind. Thus, pros expresses the intimate and special relationship that Christians will experience “at home with [pros] the Lord.” So John thus conceives the fellowship between the Logos and God.”

New Testament scholar Marvin Vincent points out:

"The preposition pros, which, with the accusative case, denotes motion towards, or direction, is also often used in the New Testament in the sense of with; and that not merely as being near or beside, but as a living union and communion; implying the active notion of intercourse...Thus John’s statement is that the divine Word not only abode with the Father from all eternity but was in the living, active relation of communion with Him."

Lenski similarly shows that pros in John 1:1b signified the inseparable communion that the distinct Person of the Word had with the Father:

The preposition pros, as distinct from, en, para, and sun, is of the greatest importance...The idea is that of presence and communion with a strong note of reciprocity. The Logos, then, is not an attribute inhering in God, or a power emanating from him, but a person in the presence of God and turned in loving, inseparable communion toward God, and God turned equally toward him. He was another and yet not other than God. This preposition pros sheds light on Gen. 1:26, “Let us make man in our image, after our likeness.”

In Romans 5:1, Paul teaches that the believer, having been justified from faith (ek pisteōs), presently and permanently has peace with God (pros ton theon). Notwithstanding the mass of biblical scholarship,17 Oneness teachers postulate a unitarian assumption denying the appropriate and natural meaning of pros in John 1:1b.18 However, the Oneness hermeneutic is flawed. Bernard and other Oneness teachers do not consider that when applied to persons, pros generally denotes (a) the intimate fellowship between them and (b) a distinction between them—every time. Nor do Oneness writers provide any New Testament examples to show the converse. Truly, the intended meaning of John 1:1b is removed by the Oneness unitarian conviction in which the Word was not pros ton theon."  (see here)

James White, Greek scholar, wrote (emphasis mine):

"The first verse itself must be examined to be understood. Transliterated into Greek the verse reads: En arche en ho logos, kai ho logos en pros ton theon, kai theos en ho logos. The verse breaks down into three clauses, each being vital to the whole. The first thing to notice is the fact that the imperfect form of eimi is used throughout the prologue in reference to the Logos. This tense, attached to the phrase "en arche" is timeless - i.e., as far back as one wishes to push the "beginning" the Word is already in existence. This is seen, for example, in the translation of the New English Bible which renders it, "When all things began, the Word already was." Today's English Version puts it, "Before the world was created, the Word already existed...." Hence, the first phrase clearly presents the eternality of the Word and hence his pre-existence.

The second phrase presents the inter-personal relationship of the Logos and God. The Greek phrase pros, translated "with," refers to the existence of communication and fellowship between the Logos and theos. The word was used to describe being "face to face" with another. Now, unless John had added the final phrase ("and the Word was God") there would have been a problem here, as the first phrase clearly presents the Logos as eternal, while the second demonstrates his distinct personality. This would create polytheism without the final phrase's emendation. At the same time, this second clause ends any chance of Sabellianism's success.

The final phrase, kai theos en ho logos, presents a syntactical arrangement in which the term theos is emphasized. At the same time, the sentence is copulative, and the presence of the article with logos simply sets it out as the subject of the sentence. Much has been said concerning the lack of the article with theos(8) but -that discussion is beyond the scope of this paper. Basically, the construction 1) avoids modalism (i.e., the Word is not said to be completely co-extensive with theos) and 2) teaches that the Word has the same nature as God (a point that Paul will reiterate in Philippians).

Verse 3 links the eternality of the Word with creatorship. "Through him all things were made; without him nothing was made that has been made." John here is intent on separating the Logos from the realm of the created - he started in the very first phrase by asserting his timeless existence and continues here by attributing to the Logos all of creation, an item that will reappear in Colossians. The only possible way to interpret these verses is to see the Logos as an eternal being who created all things.

The prologue continues by identifying the Logos with the person of Jesus Christ in 1:14. It is interesting to note that John very carefully differentiates between the Word in his absolute nature and all other things. When the eternal Word is in view, John uses en. When created things are being discussed (such as John in 1:6), the aorist egeneto is found. However, when we come to the time event of 1:14 (i.e., the incarnation), John switches from the timeless en to the aorist egeneto - the Word became flesh at a point in time in history.

Finally, in 1:18(9), John seals the case by calling Jesus the "only-begotten God," or, more accurately, the "unique God"(10) who reveals the Father, who "exegetes"(11) God to man.

These verses with which John begins his gospel are meant, in my opinion, to form an "interpretive window" through which the reader is meant to look at the words that follow. One must constantly keep the Logos in the back of the mind when interpreting the words and actions of Jesus.(12) Much of what Christ says must be understood in this light to even make much sense! His unique relationship with the Father is intelligible only in the light of his eternal preexistence with him.

Equally significant are Jesus' own "I am" sayings found in John 8:24, 8:58, 13:19 and 18:5-6. Though there is some discussion concerning the use of the phrase ego eimi in this absolute sense(13), these passages clearly show an intentional aspect to Christ's words relevant to his identity. In both 8:58 and 18:5-6, John takes pains to make sure the reader understands the impact of Christ's words on his hearers. In 13:19 we find an extremely close parallel to the LXX rendering of Isaiah 43:10, here applied to Christ by himself. One can hardly escape the significance of the Hebrew term ani hu as used by Isaiah, and its Greek translation as ego eimi. Since Christ purposefully utilized these phrases of himself, it is safe to say that he was claiming for himself the title of the "I Am" - the eternal one, YHWH."  (see here)