Only God or only Son
Whether the evangelist writes 'only God' or 'only Son', and what the reading carries.
John 1:18
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
The Jewish Encyclopedia notes that the Gospel of John and First John give the term “Son of God” a metaphysical and dogmatic significance. It says the Alexandrian Logos concept undoubtedly had a formative and dominant influence on how Jesus’ sonship is presented in the Johannine writings.
The Jewish Encyclopedia (1905), The Jewish Encyclopedia (1905), art. "Son of God" — On the Johannine use of the term
Patristic
Augustine takes “in the bosom of the Father” to mean “in the secret of the Father.” God has no bodily bosom as we do, nor is He to be thought of as sitting or girded with a belt. Because our bosom is within, the secret of the Father is called the bosom of the Father.
Augustine of Hippo, Homilies on the Gospel of John; Tractates on the First Epistle of John (NPNF1-07) — Tractate III on John i. 15-18, on the words "in the bosom of the Father"
Catholic
Haydock states no mortal in this life has a perfect union and enjoyment of God; no creature comprehends his infinite greatness. None but his only begotten divine Son is in the Father's bosom, not only by a union of grace but by a union and unity of substance and nature.
George Leo Haydock, George Haydock's Catholic Bible Commentary (1859) — Comment on John i. 18
Orthodox
Cyril says the Son is Only-Begotten God. As such, He stands outside the rest and is not counted among those who are gods by grace. Instead, He is Very God with the Father.
Cyril of Alexandria, Commentary on the Gospel According to S. John, Vol. 1 (P. E. Pusey trans., 1874) — Book I, Chapter X (on John i. 18) - the Son as Only-Begotten God
Reformed / Protestant
Calvin says the “bosom of the Father” metaphor comes from men who receive into their bosom those to whom they tell all their secrets, the breast being the seat of counsel. Thus the Son knew the Father’s most hidden secrets, showing that the Gospel lays God’s breast open to us.
John Calvin, Commentary on the Gospel According to John, Volume 1 (Calvin Translation Society) — Comment on John i. 18 (on "in the bosom of the Father")
Evangelical
John Wesley's note states that no one has seen God with bodily eyes, yet believers see him with the eye of faith. The phrase "who is in the bosom of the Father" denotes the highest unity and the most intimate knowledge.
John Wesley, John Wesley's Notes on the Bible, Notes on the Gospel According to St. John — Note on John i. 18