From the time that man fell into sin in Genesis, God worked to restore him to his original relationship with God. In fact, God was aware of the bad choices man would make even before He created him, and already had a plan in place for his redemption. Part of that plan included providing a perfect man to die for the sins of all mankind. To accomplish this, God Himself would have to become a man – something referred to in theological terms as the Incarnation, or the “becoming flesh.” The Bible tells us in Matthew 1:20 that God’s own Spirit caused the virgin Mary to conceive and bring forth a child, and that this child would be the flesh through which God would reveal Himself more fully. Quoting the Prophet Isaiah, Matthew 1:23 declares that the child’s name would be called Immanuel, which means “God with us.”
The Son of God born to Mary is different from all others referred to in the Scriptures as “sons of God,” because while their relationship to God is as the product of His creation, the Son of God is the product of God’s procreation. In other words, Jesus was the only son of God to be fathered by Him in a woman’s womb. This act of procreation is referred to in Scripture as “begetting,” and it is the reason God is not referred to in the Bible as the Father of the Son of God until the New Testament – simply because, until then, the Son of God had not been fathered.
So, while Jesus Christ was not the only one referred to in the Bible as a son of God, He bears the distinction of being the only begotten Son of God. This is made clear in John 3:16: “For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.”
The Greek word that is translated “only begotten” in our Bibles is monogenes, and it can mean “only begotten,” “only,” “only child,” “unique” or “special.” That its correct application is “only begotten” is evident from the fact that, as we have already pointed out, there are many others called “sons of God” in the Bible, so Jesus could not just be the “only Son.” Clearly, the distinction the inspired writer intended to make by using “monogenes” was the fact that Jesus was unique in that only He was actually procreated by God.
To conclude, let me reemphasize one extremely important point: while He is described as Father throughout the Bible in the sense of being the Creator, God was not identified in Scripture as the Father of the Son until there was a Son – and there was no begotten Son of God in the Scripture until Jesus was born to Mary in Bethlehem.
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Adapted from my book, Theology for the Rest of Us: Key Bible Doctrines in Everyday Language. Available at Amazon.com.