Question: "Who is the wicked one (1 John 5:19)?"
Answer: In the apostle John’s first letter, John discusses a number of characters. Of course, he talks about God and about believers—especially referring to them as brethren, little children, children, young men, and fathers. But John also makes repeated mention of the evil one or the wicked one (1 John 5:19).
John explains that the young men he was writing to have overcome the wicked one (1 John 2:13–14). John observes that Cain was “of the evil one” or the wicked one (1 John 3:12). John encourages his readers by explaining that those who are born of God are not touched by the wicked one (1 John 5:18), and he reminds them that this world is “in” the wicked one; that is, the world is in the power of the wicked one or under his control (1 John 5:19). Still, with all these cautions, John doesn’t directly identify the wicked one in his first letter. He writes as if his readers are already familiar with the identity of this wicked one.
In John’s Gospel, John recorded a prayer of Jesus in which He asks the Father to protect His disciples from the evil one, or the wicked one (John 17:15). Matthew also recorded Jesus as explaining that the wicked one snatches away the word of the kingdom (Matthew 13:19) and that those who choke the growth of the sons of the kingdom are the sons of the wicked one (Matthew 13:38). Paul adds that the wicked one attacks believers with flaming arrows (Ephesians 6:16), that the Lord will strengthen and protect His believers from the wicked one (2 Thessalonians 3:3), and that the flaming arrows of the wicked one can be extinguished by the shield of faith (Ephesians 6:16).
While it may be curious that in these contexts the wicked one is not directly named, it seems clear that this is indeed Satan. Note Paul’s description of believers being rescued from the domain of darkness and transferred to the kingdom of Christ (Colossians 1:13). Because of that transfer, believers should be focused not on the things of earth but on the things above where Christ is (Colossians 3:1–4). This world is still part of the domain of darkness, and it is governed by a prince (Ephesians 2:2) who is an enemy of believers. In Paul’s defense of the gospel before King Agrippa, he recounted his conversion, saying that Jesus had sent Paul to Jews and Gentiles “to open their eyes so that they may turn from darkness to light and from the dominion of Satan to God” (Acts 26:18). Further, in Revelation 12:11 John again records that “the brethren” overcame the accuser—specifically identified as Satan (Revelation 12:9). This corresponds with 1 John 2:13–14, which says that believers have overcome the wicked one. The one believing in Jesus overcomes the world (1 John 5:4–5), because Jesus has overcome (Revelation 5:5). Based on these contrasts of light and darkness and God’s kingdom and Satan’s dominion; and based on Jesus’ overcoming and His believers’ overcoming of Satan, it is evident that the wicked one is another title for Satan.