Now is the judgment of this world: now shall the prince of this world be cast out.
Note 8 at Joh 12:31: A future judgment of the world is still coming when the wicked will be separated from the righteous and cast into the lake of fire (see note 4 at Mr 3:29). This verse is referring to the fact that the sins of the world were about to be placed on Jesus, and He would suffer our punishment (Ro 6:23).
Jesus suffered God's punishment upon our sins, so there is no reason we should have to suffer for them too. The price for sin has already been paid by the only One who could fully pay it, and that is Jesus. All Jesus asks of us to make His redemption ours is faith in Him as our Lord (Ro 10:9).
Note 9 at Joh 12:31: The only reason Satan ever had any right to become a prince or rule over us was because we yielded ourselves to him through sin (Ro 6:16). Since Jesus "bare our sins in his own body on the tree" (1Pe 2:24), Satan no longer has power or authority over those of us who accept Jesus' gift of salvation. Satan has been "cast out," stripped of any power he had. Now he can only deceive, and if we fall for his lies, we stop God's blessings in our own lives.