Simon's "belief" was evidently only a belief in the reality of the signs and wonders performed by Philip (note Christ's rebuke of this kind of belief in John 4:48; also compare John 2:23-25). These wonders were greater than those Simon was able to perform with his sorceries (Greek mageia, from which we get our word "magic"), and he was envious. In the early Christian literature, he was called Simon Magus, and was said to be a prominent enemy of the true faith.

Source:

KJV Defenders Study Bible, by Dr. Henry Morris, Ph.D.
Publisher: Thomas Nelson
Language: English
DEF 10 ISBN 0-529-10444-x
DEF 10-1 ISBN 0-529-10445-8