I love using the ESV. It is probably my favorite translation of the Bible. Nevertheless, I do not always agree with the translators in their choice of textual basis. Generally, they follow the NA27/UBS4 text (from here on NU-Text), while I prefer the Byzantine Majority text (from here on M-Text), which is closer to the Textus Receptus (from here on TR) on which the KJV was based. While there are many instances in which the NU-Text is arguably superior to the M-Text and the TR, there are some readings where I am quite sure that the NU-Text is in the wrong, and the traditional reading is correct. One of those places is John 1:18.
"No one has ever seen God; the only God, who is at the Father’s side, he has made him known." ESVThere should be one very apparent difference here. In three of these translations, all following the NU-Text, Jesus is called "the only begotten God" (Greek: ho monogenes theos) while in the other three, He is called "the only begotten Son" (Greek: ho monogenes huios). This difference is somewhat significant. First, though, let's look at the manuscript evidence for the readings.
"No one has ever seen God, but God the One and Only, who is at the Father's side, has made him known." NIV
"No one has seen God at any time; the only begotten God who is in the bosom of the Father, He has explained Him." NASB
"No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared him." KJV
"No one has seen God at any time. The only begotten Son, who is in the bosom of the Father, He has declared Him." NKJV
"No one has ever seen God. The One and Only Son—the One who is at the Father's side—He has revealed Him." HCSB
The reading "the only begotten God" has notable support. It appears in P75, a papyrus from the second or third century, Codex Sinaiticus, a large manuscript from the fourth century, Minuscule 33, and a Coptic translation. A similar reading, only lacking "the," is found in P66, Codex Vaticanus, Codex Ephraemi Rescriptus, and Codex Regius. Most modern scholars hold Sinaiticus, Vaticanus, and Ephraemi in high regard.
The alternate reading, "the only begotten Son," has majority support. While it appears in fewer early manuscripts, it does appear in Codex Alexandrinus, a fifth century manuscript which is well valued. The entire list is as follows: C3, K, X, Δ, Θ, Π, 063, 0234, f1, f13, 28, 565, 700, 892, 1009, 1010, 1071, 1079, 1195, 1216, 1230, 1241, 1242, 1253, 1344, 1365, 1546, 1646, 2148, Byzantine majority, Adysh Gospels. If you don't know what all these things mean, just know this: this group includes the vast (and I mean very vast) majority of existing New Testament manuscripts containing John 1:18, representing manuscripts of every text-type and from a wide geographic area.
So, the "only begotten God" reading has limited support, only from a handful of Alexandria manuscripts, however old they may be. While the "only begotten Son" reading seems to show up slightly later, it has very vast support, over a wide area and full range of text-types. Personally, I tend to think this is strong evidence for "Son," but many argue that the manuscripts which support "God" are so superior as to offset this. Maybe. Next is the internal evidence, which should make a profound impact, as well.
Before actually tackling the internal evidence, let me explain the term "only begotten," since it also appears as "one and only" or simply "only." The Greek word monogenes is a combination of two Greek roots, namely monos (meaning "one" or "only") and gennao (meaning "begotten" or "born"). So, in a maximally literal rendering, monogenes should be "only begotten." However, the most literal rendering of compound words isn't always the most accurate to convey its connotation. Other factors are involved in monogenes. For example, the word is used in Hebrews 11:17 to describe Isaac in relation to Abraham. Now, we know that Isaac wasn't Abraham's only begotten son in a strict sense. Ishmael was also Abraham's son. Is Hebrews 11:17 wrong? No. There are two ways this difficulty can be resolved. Either monogenes means something less literal such as "unique" or "one of a kind," or it can also be used in reference to Jewish inheritance laws and customs regarding lineage, in which case Isaac was treated legally as Abraham's only son. Which of these is most likely? Well, first we should note that the word still includes "begotten." So, while many modern translators want to removal that element and make the word simply an expression of uniqueness or special favor, that is really unwarranted. Isaac was still begotten of Abraham, and indeed Jesus was said to have been begotten of the Father. Therefore, it makes sense to say that monogenes still implies begetting, but can also involve legal connotations.
The reading "only begotten Son" is obviously the easiest reading. John used the exact same term several times, three other times in his Gospel and once in 1 John. "The only begotten God" is never used anywhere else in the New Testament. Oddly, though, many scholars argue that this means "God" is the correct reading. Why? They say that, since the reading is so odd, it is unlikely to have been made up by a scribe. Instead, "God" would have been the original reading, and a scribe later, either accidentally or on purpose, changed it to "Son." This argument seems to have some validity, but a counter can easily be made. The fact is that there is probably a reason "God" is the harder reading: it's not supposed to be there. See, people often set up a double standard with Greek and English. Imagine if I wrote a book where I referred to a certain character as "King of Maine" quite often. However, the original book was lost and you found a handwritten copy of it. If you were reading and suddenly, after seeing "King of Maine" several times, saw the phrase "King of Main," which would you think is correct? Am I still calling the character the King of Maine and this is a mistake, or did I originally write King of Main for some reason? The most reasonable conclusion is that the common title is the correct reading. Also note that, like in this scenario, the difference between "God" and "Son" in the Greek manuscripts would only be one letter due to a shorthand used to refer to sacred names called nomina sacra. "God" is ΘΣ, and "Son" is ΥΣ. That's an easy enough mistake either way, but I think the common title is more likely. If a scribe made a mistake in that verse, it should be noted that the sentence starts with theos, therefore drifting eyes could easily run to that word when copying.
I believe the theological evidence is also significant. The term "only begotten God" is theologically unnerving. There is only one God, as we know. Calling Jesus "the only begotten God" seems to imply the existence of another God, unbegotten, such as early heretics said. The title appears to make Jesus and the Father separate deities. If they were the same God, then the Father would also seem to have been begotten, which is entirely absurd! We've already seen that there is no justification for removing the begetting aspect from monogenes, so it makes no sense to ascribe the term to the entire Godhead or God the Father, which are the only ways the term theos is ever used in the New Testament. Considering the theology of the Incarnation, I believe that the "God" reading is highly suspect.
Finally, I will present evidence from the writings of the early church fathers, which prove conclusively the antiquity, if not validity, of the "Son" reading. Ignatius, Irenaeus, Clement, Tertullian, Origen, Hippolytus, Archelaus, and Alexander, who all wrote within the first, second, and third centuries, attest clearly to the reading, "only begotten Son." These very early witnesses affirm that the "God" reading isn't necessarily older or better. In fact, one of the early external witnesses to the "God" reading is the Arian Creed, a heretical document outlining a view of Jesus similar to that of Jehovah's Witnesses today. It seems to me, in light of everything mentioned so far, that the traditional reading of "only begotten Son" is very likely to be the correct reading.