For a man who has been so prominent in Christian thinking throughout the centuries John is a strangely shadowy figure. In the Gospels and Acts he is almost invariably accompanied by someone else and the other person is the spokesman (there is an exception when John tells Jesus that he forbade a man to cast out demons; Luke 9:49). He is often linked with Peter and with his brother James, and these three were specially close to Jesus (Matt. 17:1; Mark 14:33; Luke 8:51). He and James were called "sons of thunder" (Boanerges; Mark 3:17), which perhaps points to the kind of character revealed in their desire to call down fire from heaven on people who refused to receive Jesus (Luke 9:54).
We learn more from the writings linked with his name. The Fourth Gospel as it stands is anonymous, but there is good reason for thinking that John wrote it and that he was the beloved disciple who leaned on Jesus' breast at the Last Supper (John 13:23) and to whom the dying Jesus commended his mother (John 19:26-27). The impression we get is that John had entered into the mind of Jesus more than any of the other disciples had.
From his Gospel we learn a good deal about the Father and, indeed, it is to John more than anyone else that Christians owe their habit of referring to God simply as "the Father." John uses the word "father" 137 times (which is more than twice as often as anyone else; Matthew has it 64 times, Paul 63). No less than 122 refer to God as Father, a beautiful emphasis which has influenced all subsequent Christian thinking. John also tells us that this God is love (I John 4:8, 16), and love is an important topic in both his Gospel and his epistles. We know love in the Christian sense because we see it in the cross (John 3:16; I John 4:10); it is sacrificial giving, not for worthwhile people, but for sinners.
The Father is constantly active (John 5:17); he upholds his creation and brings blessing on those he has made. He is a great God whose will is done, particularly in election and salvation. "No one can come to me," said Jesus, "unless the Father who sent me draws him" (John 6:44); and again, "You did not choose me, but I chose you" (John 15:16; cf. 8:47; 18:37).
The book of Revelation was written by John (Rev. 1:1-3), though which John is not specified. But there is good reason for seeing it as coming from John the apostle and as stressing an important aspect of Johannine thought, namely that of divine sovereignty. It is easy to get lost in a strange world of seals, trumpets, bowls, and animals with unusual numbers of heads and horns. But this is not the important thing. Throughout this book God is a mighty God. He does what he wills and, though wickedness is strong, in the end he will triumph over every evil thing. There is a great deal about the wrath of God in Revelation (and something about it in the Gospel), which brings out the truth that God is implacably opposed to evil and will in the end overthrow it entirely.
Throughout the Johannine writings there is a good deal of attention given to Christology. The Gospel begins with a section on Christ as the Word, a passage in which it is clear that God has taken action in Christ for revelation and for salvation. Christ is "the Savior of the world" (John 4:42), and this is brought out when he is referred to as Christ (= Messiah), Son of God, Son of man, and in other ways. They all depend in one way or another on the thought that God is active in Christ in bringing about the salvation he has planned. John has an interesting use of terms like "glory" and "glorify," for he sees the cross as the glorification of Jesus (John 12:23; 13:31). Suffering and lowly service are not simply the path to glory; they are glory in its deepest sense. This striking form of speech brings out the truth that God is not concerned with the kind of thing that people see as glorious. The whole life of Jesus was lived in lowliness but John can say, "We have beheld his glory" (John 1:14).
John's treatment of the miracles is distinctive. He never calls them "mighty works" as do the synoptists, but "signs" or "works." They point us to significant truth, for God is at work in them. "Work" may be used of Jesus' nonmiraculous deeds as well as those that are miraculous, which suggests that his life is all of a piece. He is one person; he does not do some things as God and others as man. But all he does is the outworking of his mission, a thought which means much to John. There are two Greek words for "to send," and John's gospel has both more frequently than any other book in the NT. Mostly he uses the words to bring out the truth that the Father sent the Son, though there are some important passages linking the mission of his followers with that of Jesus (John 17:18; 20:21). Being sent means that Jesus became man in the fullest sense, as is brought out by his dependence on the Father (cf. John 5:19, 30) and by statements about his human limitations (e.g. John 4:6; 11:33, 35; 19:28). John's Jesus is fully divine, indeed, but he is also fully human.
John tells us more about the Holy Spirit than do the other evangelists. He is active from the beginning of Jesus' ministry (John 1:32-33), but the full work of the Spirit among man awaited the consummation of Jesus' own ministry (John 7:39). The Spirit is active in the Christian life from the beginning (John 3:5, 8) and there are important truths about the Spirit in Jesus' farewell discourse. There we learn among other things that he is "the Spirit of truth" (John 14:16-17), that he will never leave Jesus' people (John 14:16), and that he has a work among unbelievers, namely that of convicting them of sin, righteousness, and judgment (John 16:8).
The Spirit is active in leading Christians in the way of truth (John 16:13), and John has a good deal to tell us about the Christian life. He speaks of "eternal life," which seems to mean life proper to the age to come, life of the highest quality (cf. John 10:10). Entrance into life is by believing, and John uses this verb 98 times (though never the noun "faith"). Believers are to be characterized by love (John 13:34-35). They owe all they have to the love of God, and it is proper that they respond to that love with an answering love, a love for God that spills over into a love for other people. This receives strong emphasis in I John. John emphasizes the importance of light (for believers are people who "walk in the light"; I John 1:7) and of truth. Jesus is the truth (John 14:6) and the Spirit is the Spirit of truth (John 14:17). To know the truth is to be free (John 8:31-32).
John's is a profound and deep theology, though expressed in the simplest of terms. It sets forth truths which no Christian can neglect.
(Elwell Evangelical Dictionary)
J. E. Davey, The Jesus of St. John; W. F. Howard, Christianity According to St. John; R. Kysar, The Fourth Evangelist and His Gospel; C. F. Nolloth, The Fourth Evangelist; N. J. Painter, John: Witness and Theologian; S. S. Smalley, John: Evangelist and Interpreter; D. G. Vanderlip, Christianity According to John.