The doctrine of trinity states that the Son is eternal, meaning He had always existed in the flesh.
However, the Holy Bible teaches that while the idea of Sonship (plan of Salvation) was from before the foundation of the world, the actual fulfilment of God manifesting in the flesh had a definite beginning in history. Before God manifested in the flesh, He had always existed in the Spirit.
Gal 4:4 But when the fulness of the time was come, God sent forth his Son, made of a woman, made under the law,
This verse declares that Sonship came into existence only “when the fulness of the time was come”. Also, the Son was “made under the law” meaning that the flesh came into existence only after the law was given by Moses.
Again, the Bible points out,
Heb 1:5 For unto which of the angels said he at any time, Thou art my Son, this day have I begotten thee? And again, I will be to him a Father, and he shall be to me a Son? 6 And again, when he bringeth in the firstbegotten into the world, he saith, And let all the angels of God worship him.
The statement made here is “I will be to him a Father” invoking the future tense. Not “I am already his Father”. Also, the verse declares “He shall be to me a Son”.
1 Tim 3:16 And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory.
The Bible says that all the angels in Heaven cover their faces in the presence of the Almighty God. However, when He was born in the flesh, they beheld His face.
On a concluding note, the office of Sonship has a definite beginning in history, and as discussed earlier, a definite ending in the future (after death has been destroyed). However, remember that God has always existed in the Spirit, even before manifesting in the flesh.