John means "grace of God", or one who has or has been given grace. Which theologian has ever been given the ability to penetrate the hidden mysteries of the Supreme Good, and teach them to human minds? Because Matthew and Luke described the birth of the Savior according to the flesh, John was silent about that, and begins with theology and from Jesus' divinity. That aspect no doubt was reserved for this outstanding man by the Holy Spirit.
Since the Gospel is the summit of all Scripture, John excels among the writers of the Gospels in treating the depths of the divine mysteries. He preached the word of God without any writing from the time of the Lord's Ascension for 65 years, until the end of the reign of Domitian. But, after Domitian was killed, with the permission of Nerva, he returned from exile to Ephesus, the bishops of Asia compelled him to write against the heretics about the divinity of Christ which is coeternal with the Father, against the heretics who denied that Christ existed before Mary.
If you carefully observe, you can see that Matthew, Mark, and Luke concentrated on those worldly deeds and sayings of the Lord which are most important for living the present life well; thus they were concerned with the active life. But John narrated many fewer events of the Lord, but put his energy into writing about His words, especially those that teach about the unity of the Trinity and the happiness of eternal life. Thus the three animals symbolizing the other three Evangelists, i.e., the lion, man, and bull, walk on the earth, because these three Evangelists are most concerned with what Christ did in the flesh, and about the commands which he gave mortal men for right living in this life. John, however, flies like an eagle above the clouds of human weakness, transcending all corners of the earth, the air, the stars, all choirs and legions of angels, and looks at the light of unchangeable truth with the sharpest and firmest eyes of the heart. For if he did not transcend all those created things, he would not reach Him through whom all things were made. For he focused mostly on the divinity of the Lord, in which He is equal to the Father, and in his Gospel he preached that as fully as he thought necessary for people to understand.
O God, who through the blessed Apostle John have unlocked for us the secrets of your Word, grant, we pray, that we may grasp with proper understanding what he has so marvelously brought to our ears. Through our Lord Jesus Christ, your Son, who lives and reigns with you in the unity of the Holy Spirit, one God, for ever and ever.
Catena Aurea (St. Thomas Aquinas)
Church History, Book III (Eusebius of Caesarea)
Commentary on the Gospel of John (St. Alcuin of York)
The Harmony of the Gospels, Book I, Ch. 4 (St. Augustine)