'The Word was God'
The prologue's grammar and translation, the Trinity's proof text, and the readings that render it indefinitely.
John 1:1-18
1In the beginning was the Word, and the Word was with God, and the Word was God.
2The same was in the beginning with God.
3All things were made through him. Without him, nothing was made that has been made.
4In him was life, and the life was the light of men.
5The light shines in the darkness, and the darkness hasn’t overcome it.
6There came a man, sent from God, whose name was John.
7The same came as a witness, that he might testify about the light, that all might believe through him.
8He was not the light, but was sent that he might testify about the light.
9The true light that enlightens everyone was coming into the world.
10He was in the world, and the world was made through him, and the world didn’t recognize him.
11He came to his own, and those who were his own didn’t receive him.
12But as many as received him, to them he gave the right to become God’s children, to those who believe in his name:
13who were born not of blood, nor of the will of the flesh, nor of the will of man, but of God.
14The Word became flesh, and lived among us. We saw his glory, such glory as of the one and only Son of the Father, full of grace and truth.
15John testified about him. He cried out, saying, “This was he of whom I said, ‘He who comes after me has surpassed me, for he was before me.’ ”
16From his fullness we all received grace upon grace.
17For the law was given through Moses. Grace and truth were realized through Jesus Christ.
18No one has seen God at any time. The one and only Son, who is in the bosom of the Father, has declared him.
What each tradition says
Rabbinic / Midrashic
Kaufmann Kohler states that the Memra as a cosmic power furnished Philo the cornerstone on which he built his semi-Jewish philosophy. Philo's divine thought, image, first-born son, archpriest, intercessor, paraclete, and arch type of man paved the way for the Christian conceptions of the Incarnation and the Trinity.
Kaufmann Kohler (1843-1926), The Jewish Encyclopedia, vol. 8, s.v. 'Memra' (New York: Funk & Wagnalls, 1904) — section 'The Logos' — concluding paragraph
Patristic
Augustine warns that when we hear “In the beginning was the Word,” we must not imagine something worthless, as we might with human words. Instead, he says, we must think of what is declared: “The Word was God.”
Augustine of Hippo (354-430), Lectures or Tractates on the Gospel According to St. John, Tractate I, in NPNF ser. 1, vol. 7 (ed. Philip Schaff, 1888) — Tractate I on John 1:1-5, section 10 (on verse 1)
Catholic
Walter Drum states that the clause theos en ho logos means “the Word was God.” He says this meaning is driven home by the following verse, “All things were made by him,” so the Word is the Creator of all things and true God.
Walter Drum, The Catholic Encyclopedia, vol. 7, s.v. 'The Incarnation' (New York: Robert Appleton Company, 1910) — Part I, section 'New Testament proofs' — the paragraph on John 1:1
Orthodox
John of Damascus says that God is everlasting and perfect. He therefore holds that God will have His Word subsistent in Him, everlasting and living, and possessed of all the attributes of the Begetter.
John of Damascus (c. 675-749), An Exact Exposition of the Orthodox Faith, in NPNF ser. 2, vol. 9 (trans. S. D. F. Salmond; Edinburgh, 1899) — Book I, ch. 6 ('Concerning the Word and the Son of God: a reasoned proof')
Reformed / Protestant
Calvin reads the prologue’s introduction as asserting the eternal divinity of Christ. Its purpose is to inform us that Christ is the eternal God, who was manifested in the flesh.
John Calvin (1509-1564), Commentary on the Gospel According to John, vol. 1 (trans. William Pringle; Edinburgh: Calvin Translation Society, 1847) — Commentary on John 1:1-5
Evangelical
Wesley takes “the Word was God” to mean supreme, eternal, and independent. There was no creature in respect of which he could be styled God in a relative sense; therefore he is styled God in the absolute sense.
John Wesley (1703-1791), Explanatory Notes upon the New Testament (1755); CCEL's 'Wesley's Notes on the Bible' — Note on John 1:1