The Gospel of Nicodemus (also referred to as The Acts of Pilate) is a purported gospel from the days of the early church. Though the book is named after Nicodemus, the Pharisee who appears in the Gospel of John, it was not written by Nicodemus. The book is actually a conglomeration of several texts, probably collected slowly over time. Scholars date the completed Gospel of Nicodemus to sometime in the mid-fourth century.
The Gospel of Nicodemus contains two parts: the first, often called The Acts of Pilate, deals with Jesus’ passion; and the second deals with Jesus’ supposed descent into hell after His death. In part one the Roman governor hears various testimonies for and against Jesus before giving in to the demands of the Jewish leaders to have Jesus executed. After the resurrection, Pilate receives evidence that Jesus is alive again, despite the attempts of the Sanhedrin to cover it up. Part two of the Gospel of Nicodemus is an appendix to The Acts of Pilate and is narrated by two characters named Leucius and Karinus. These were supposedly two of the men who were raised from the dead by Jesus at His resurrection (see Matthew 27:52–53). These two men, Leucius and Karinus, give a firsthand report of what happened in hell when Jesus descended and set the captives free. According to the Gospel of Nicodemus, when Leucius and Karinus finished giving their testimony to the Jewish authorities in Jerusalem, they vanished from this world.
There’s absolutely no reason to think that the Gospel of Nicodemus is a genuine gospel. The book doesn’t date to before the fourth century, at least three hundred years after Jesus died and rose again. There are no references to the Gospel of Nicodemus by any major early Christian writers such as Irenaeus or Eusebius. Further, the book claims to record a number of events, such as the conversion of Pilate to Christianity, that are not corroborated in the Bible. There is no mention of men named Leucius and Karinus in the Bible. The Gospel of Nicodemus appears to have been written long after the Bible was completed, too late to have been written by Nicodemus, Pilate, or any other purported author.
That said, the Gospel of Nicodemus was a well-known text in the Middle Ages. Medieval ideas such as the Harrowing of Hell (the teaching that, between the Crucifixion and the Resurrection, Jesus went to hell, broke down its gates, and delivered the righteous from captivity) are largely drawn from the Gospel of Nicodemus. So, while the book may be an interesting read for cultural or academic reasons, there’s no reason to take it seriously as theology or history. The Gospel of Nicodemus is pseudepigraphal and not a “lost” book of the Bible.