The Gospel of John was written by one of Jesus’ 12 apostles. He was the youngest of the 12 and “the one whom Jesus loved” (cf. Jn 13:23; 19:26; 20:2; 21:20,24). With James and Peter, he was part of the “inner circle” of apostles who were closest to Jesus and who alone were privileged to witness important moments in Jesus’ ministry, such as the Transfiguration (cf. Mk 9:2). John was the only apostle who stayed with Jesus at the Crucifixion, and Jesus gave His mother Mary into the care of John shortly before He died. Finally, besides the Gospel, John also wrote three letters and the Book of Revelation, which are found in the New Testament.
Scholars date the composition of the Gospel to around 90 AD. Since the Gospel shows detailed knowledge of Jewish feasts, a familiarity with Palestinian geography, and makes many allusions to the Temple, it was probably written for Jews and Jewish Christians living throughout the Mediterranean world. As for the purpose of the Gospel, John specifically says, “These are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in his name” (Jn 20:31). John is also writing in order to “fill in the blanks” left by the Gospels according to Matthew, Mark, and Luke. John informs us of many facts and events that are not found in the other Gospels.
John’s Gospel is a truly beautiful work, rich in meaning and deeply spiritual. John himself is symbolized by an eagle because his Gospel soars to new heights of contemplation upon the divinity of Christ. John proves with certainty that Jesus is God. His Gospel is also very familial: it focuses on God as a family of Persons — Father, Son, and Holy Spirit — and upon mankind as initiated into that family through faith in Christ. In relation to the other Gospels, the Trinity finds its fullest expression in John’s Gospel. Life, truth, light, glory, and “bearing witness” are common themes found throughout. Faith and love are also very important.
Finally, John’s Gospel is very sacramental. He speaks explicitly of Baptism (cf. Jn 3:3,5), Eucharist (cf. Jn 6), and Confession (cf. Jn 20:21-23). Jesus gives marriage new sanctity with His presence at the wedding feast at Cana (cf. Jn 2:1-11). John speaks indirectly of Confirmation by including Jesus’ promise to send the Holy Spirit to the Apostles (cf. Jn 14:26; 16:13). The priesthood is even seen in Jesus’ priestly prayer in Jn 17, where Jesus intercedes as High Priest to the Father on our behalf and offers himself as a holy victim.
This is truly an amazing work, and I hope you will set about reading it and studying it if you haven’t already.