First, Satan rejected God and his divine plan for him and the angels, not vice versa (CCC 391-93).
In addition, as part of the drama of his divine plan, God permits Satan and his demonic minions to test man, whom God similarly gives free will to choose to trust and serve him (God) or not. And yet, to be clear, God does not abandon mankind to Satan and his demonic associates.
Indeed, while God allows the devil to be active on earth (1 Pet. 5:8-9), he doesn’t leave us to fend for ourselves, sending his Son as the Redeemer and Savior of all mankind (see John 3:16-17; 1 Tim. 2:4). Indeed, Jesus promised to be with us until he returns at his Second Coming (see Mt. 28:18-20), and a key way he does so is through the celebration of the sacraments, particularly the Eucharist, which he commanded us to do “in remembrance of him” (Luke 22:19-20).
The salvific drama plays out in each of our personal lives (CCC 407ff.). Given original sin and the effects thereof (see CCC 402ff.), Satan has a certain dominion over the world (see Jn. 12:31; 2 Cor. 4:4; Eph. 2:2). In Christ, we can overcome Satan (see Col. 1:13-14), but the devil can still seek to undermine us (see Jas. 4:7-8). So we still need to carry our crosses during our earthly sojourn (see Matt. 7:13-14; 16:24-25).
Finally, Jesus never abandons a lost sheep (see John 10:1-21; Luke 15:3-7), although a sheep may definitively reject Christ at the end of his life (see CCC 1033-1037). Satan’s reign will definitively end at Jesus’ Second Coming.