The Catechism in Questions & Answers

100. What does it mean when it says that Jesus Christ is God’s “only begotten Son”?

The designation of Jesus as God’s “only begotten Son” means that Jesus Christ, the Son of God, is unique and eternal. The Son of God is not part of the creation like mankind, nor can He be compared to the angels, who have a beginning. He is without beginning and without end. He is God, and thus of the same essence as the Father and the Holy Spirit. Thus He has always been—that is, even before all creation—in fellowship with the Father and the Holy Spirit (pre-existence).

The term ‘pre-existence’ is derived from the Latin words prae and existentia, which mean “before” and “existence”, respectively. In relation to Jesus Christ, pre-existence means that the Son of God is from eternity, which means that He has always existed, even before the creation and His incarnation.