The four Gospels in our Bible had all been written by the end of the first century. Apparently no other gospels were written by this time. By the last 20 years of the second century, when Irenaeus the bishop of Lyon was writing, the four Gospels had been widely and firmly established for some time as the only ones accepted by mainstream Christianity. However, many sections of the church did not use all of them.
Irenaeus argued against accepting other gospels, such as the Gospel of Truth, alleged to have been written by the Gnostic teacher Valentinus. He said it had only recently been written and "did not agree in any respect with the Gospels of the apostles." This gospel is a homily or meditation and does not resemble our biblical Gospels in telling of the activities and teaching of Jesus, including His appearances after Easter. The same is true of the Gospel of Philip, an anthology of sayings from the mid-fourth century, as well as the second-century Greek Gospel of the Egyptians, about which we know little except that it was apparently a collection of sayings. The Gospel of Thomas, which also contains a collection of sayings of Jesus (some of which may be historically authentic) along with minimal narrative material, has been argued to be early. However, because of parallels with literature of this period, many date it late in the second century. More fanciful gospels include the Infancy Gospel of Thomas, with its miracles conducted by the child Jesus, ending with the story from Luke of the 12-year-old Jesus in the temple.
Other gospels approximate those in the NT. For example, the now largely missing Gospel of Peter came from the middle of the second century. From the fragmentary evidence we have, it told of the trial of Jesus, His crucifixion, and His appearing to a group of His followers. Also, the Gospel of the Ebionites, from Syria in the same period, is a harmony of Matthew, Mark, and Luke. Later in the century Tatian produced a widely used harmony of all four Gospels, the Diatessaron, which was highly valued particularly in Syria. From papyrus fragments we also have evidence of a handful of other gospels from as early as the second century. A letter of Clement of Alexandria (c. 150-215) discovered in 1958, which tells of a "secret gospel" of Mark, may be a modern forgery.
The Gospel of Hebrews, written before the mid-second century, perhaps in Egypt for Greek-speaking Jewish Christians, was the only gospel apart from the four in our Bible that was ever considered part of the legitimate Scriptures by sections of orthodox Christianity. The few remaining quotations of it show that it probably began with Jesus' preexistence and included His descent from heaven and subsequent birth. Jesus describes Himself as the son of the Holy Spirit and reports His temptation. There are also examples of His teaching. During the Last Supper, James the brother of Jesus says he will not eat again until he has seen the risen Jesus. There was probably a story of the burial of Jesus, and those who guarded the tomb may have witnessed the resurrection. As anticipated, there is a story of Jesus appearing to James, reinforcing his importance to this gospel. Gnostic characteristics, divergence from the biblical Gospels, and lack of any connection with an apostle may account for its eventually being excluded from the NT by mainstream Christianity.
Extracted from the Apologetics Study Bible.