So what are we to understand? I believe we are to understand that we are born captives into this world (“Because of original sin, we are born without grace in our souls”) for satan rules this world, and we are born into a world of sin and born as sinners ourselves who are ruled by satan, and so we are born as slaves to him (this world). That is why Christ says if we are not sons of God then we are sons of the devil. That is why Christ came to free us, with His own Body and Blood He paid the price (the price of a slave of satan which is death – but Christ not only died for us but rose again that we may too rise from death to have eternal life) to set us free. For those who accept Christ’s sacrifice for their sins (through baptism and perseverance – see catholic teachings), they are no longer slaves to this world and the ruler of this world but reborn, born again to salvation, now called the children of God. So, we can not be of our own, because in this world we belong as children of the devil as we are born into this world captives of sin. But through Christ we are free, if we repent and are obedient rejecting this world and its ruler (for if you are not obedient to God, you are obedient to the devil your father).

Luke 4:17-21
17 And the book of the prophet Isaiah was handed to Him. And He opened the book and found the place where it was written, 18 “THE SPIRIT OF THE LORD IS UPON ME, BECAUSE HE ANOINTED ME TO PREACH THE GOSPEL TO THE POOR. HE HAS SENT ME TO PROCLAIM RELEASE TO THE CAPTIVES, AND RECOVERY OF SIGHT TO THE BLIND, TO SET FREE THOSE WHO ARE OPPRESSED, 19 TO PROCLAIM THE FAVORABLE YEAR OF THE LORD.” 20 And He closed the book, gave it back to the attendant and sat down; and the eyes of all in the synagogue were fixed on Him. 21 And He began to say to them, “Today this Scripture has been fulfilled in your hearing.”