For as the Father has life in himself, so he has granted the Son to have life in himself.
The moon, the planets and, as we have seen from space exploration, the earth itself, have only reflected light from the sun. So it is with life on earth; we human beings together with every creature whether plant or animal or microbe only have life because of being created by one with life in himself. The Father has it and so does the Son.
Some theologians talk about the 'eternal generation of the Son'. It all depends what they mean by this, but it should imply that this granting of life to the Son was not something that began at the the incarnation, since John has already told us that 'all things were created by him.'
I like JC Ryle on this: The privileges of a true Christian are greatly underrated by many... One of a true Christians's treasures is the 'presentness' of his salvation. It is not a far distant thing which he has to have at last, if he does his duty and is good. It is his own in title the moment he believes. He is already pardoned, forgiven and saved - though not in heaven.
Another of his treasures is the 'completeness' of his justification. His sins are entirely removed, taken away and blotted out by Christ's blood. He may look forward to judgement without fear. He shall stand without fault before the throne of God.
The third treasure is the entire change in his relation and position towards God. He is no longer as one dead before him - dead legally, like a man sentenced to die, and dead in heart. He is alive unto God. He is a new creature. Old things are passed away and all things are become new.