Jesus tells us that the bad fish are thrown into the fire. John the Baptist says this in a slightly different way in Matthew 3:12: "[Jesus] will burn up the chaff with unquenchable fire." This principle appears somewhat differently in the Parable of the Sheep and Goats (Matthew 25:31-46): Christ is Judge, and He sets the sheep on His right hand and the goats on His left. He judges that the sheep can enter eternal life, while the goats receive the destructive judgment of fire.
Although a final judgment is coming for the world, the church is now under God's judgment (I Peter 4:17; Revelation 11:1-2). Not only is the sentence coming, but our conduct and growth are also currently being judged - Christ is evaluating whether we meet His high standards. Ultimately, everyone is judged the same way, according to the same standard, by the same criteria. The "bad fish" among us are not ours to judge, but Jesus, the righteous Judge, has promised to judge with equity (Psalm 98:9).
Matthew 13:50 says they are thrown "into the furnace of fire." A similar thing occurs in the Parable of the Wheat and the Tares: At the end of the age, the tares will be gathered and thrown into the furnace (verses 30, 41-42). The emphasis in the Parable of the Tares is on the wicked and their evil works and their subsequent judgment. However, in the Parable of the Dragnet, instead of highlighting the wickedness, Jesus focuses on the process of judgment, not necessarily on condemning evildoers. Some people are condemned for doing wicked things, but others are saved and rewarded for doing the good works assigned to them. God's calling is first impartial, and then His judgment is absolutely fair. The wicked will get only what they deserve.
God's "catch" is the church, a chosen cross-section of the entire world; He casts a wide net. However, once those He calls accept Jesus Christ, God does show Himself partial to the "good fish" - those who love Him, obey Him, serve others, grow, and produce spiritual fruit. In the process of salvation, God judges whether we are good, useable fish or substandard fish fit only for the fire. He judges us according to how we measure up against His standard of righteousness, "the perfect man, . . . the measure of the stature of the fullness of Christ" (Ephesians 4:13). God throws His net into the world and drags us in, and if we are unwilling to comply with His holy standard, our eternal judgment will be to be discarded in the fire.
Presently, the church's function is not judicial but declarative. On the one hand, the church is responsible to warn sinners of the dire consequences of sin and of the time of God's judgment coming upon all humanity. On the other hand, we are to witness of God's way of life, as well as to proclaim Christ's return and the establishment of God's wonderful, benevolent government here on earth. That is good news!