Simon was the Samaritan sorcerer who professed conversion to Christianity and sought to buy an apostleship. The Bible records this historic event in Acts 8:9-24.
In spite of Peter's stinging rebuke (verses 20-23), tradition and various legends say Simon presented himself as a Christian apostle, particularly in Rome. He invented a new religion by blending his own version of the doctrine of grace with elements of the old Babylonian mysteries and attaching Christ's name to it. The International Standard Bible Encyclopedia comes to this conclusion about him based on what is available from history:
The early Christian writers took this view [from Irenaeus, c. AD 125-202], and regarded Simon Magus as the founder of Gnosticism. Perhaps they were right, "but from the very little authentic information we possess, it is impossible to ascertain how far he was identified with their tenets" (Alford, New Testament, II, 86). In the midst of the various legends regarding Simon, it may be that there is a substratum of fact, of such a nature that future investigation and discovery will justify these early Christian writers in their judgment, and will show that Simon Magus is not to be overlooked as one of the sources from which Gnosticism sprang. The exact origin of Gnosticism is certainly difficult to trace, but there is little or no indication that it arose from the incidents narrated in Acts 8. It cannot be denied that a connection is possible, and may have existed between the two, that is between Simon Magus and some of the gnostic heresies; but the facts of history show widespread tendencies at work, during and even before the Apostolic age, which amply account for the rise of Gnosticism.
There are veiled references to Simon's false Christianity and similar heretical sects in the New Testament. Jude 4, for example, is rather pointed against Simon's principal doctrine—the heresy that one does not have to obey God's laws after conversion. John, the apostle who completed the Bible, placed great emphasis on Christians keeping God's commandments (I John 2:3-6).
John's phrase about those "who say they are Jews, and are not, but lie" (Revelation 3:9) may have its first-century basis in Simon's Samaritan counterfeit of true Christianity. Josephus, a Jewish historian of the first century, mentions that Samaritans would falsely claim to be Jews when they thought it was to their advantage to do so (Antiquities of the Jews, 9.14.3; 9.8.6). As prophecy, however, this phrase clearly has deeper spiritual meaning.
Additional information about Simon Magus can be found in these reference works: The eleventh edition of the Encyclopaedia Britannica, Schaff's History of the Christian Church; Hastings' Dictionary of the Apostolic Church and Dictionary of the Bible; Dictionary of Christian Biography; and the Encyclopaedia Biblica.