Do you understand eternal means 'to be without beginning or end'? So no, God could not create an eternal being because that would require Him creating Himself which, cannot happen..
So are you saying the Son of God is not eternal? or the Holy Spirit is not eternal?
Since the eternal God brought them into existence then they had a beginning of existence.
They are distinct from one another in their relations of origin: "It is the Father who generates, the Son who is begotten, and the Holy Spirit who proceeds."
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The divine persons do not share the one divinity among themselves but each of them is God whole and entire:
https://www.vatican.va/archive/ENG0015/__P17.HTM
Actually, I understand that the eternal has always existed, and does not change in form or nature, which is the reason to believe that the eternal will always exists.
People lived well beyond 120 years after the flood. The verse in Genesis 6:3 is not about setting a limit on life span of man but on setting a limit on the number of years man will have before judgment is executed by means of the flood.
That doesn't hold water since it was revealed after men began to multiply upon the face of the earth in the beginning and before the next generation was born.
Besides, after the LORD brought the cloud over the earth that covers it like a garment, being the firmament, that he made from the waters that covered the face of the earth that he said the waters would never cover the earth again.
And it shall come to pass, when I bring a cloud over the earth, that the bow shall be seen in the cloud: Gen 9:14
Where wast thou when I laid the foundations of the earth? declare, if thou hast understanding.
When I made the cloud the garment thereof, and thick darkness a swaddlingband for it,
Job 38:4&9
And I will remember my covenant, which is between me and you and every living creature of all flesh; and the waters shall no more become a flood to destroy all flesh. Gen 9:15