A god is a son of God and a son of God is one whose very existence was because of God. So firstborn as a son of God means being the only one of your kind until others are brought into existence by God in that same way. As firstborn over all creation there were no other sons of God for a while when Jesus was brought into existence by God before the world began. As one born of a virgin there was only Jesus brought into existence by God this way as an only begotten son of God. As firstborn from the dead there are no other resurrected immortal humans for a while when Jesus is the only one brought again into living existence from the dead until others too are so raised.
