Act.8:13, "Then Simon himself believed also: and when he was baptized, he continued with Philip, and wondered, beholding the miracles and signs which were done," is qualified by v21-23, "Thou hast neither part nor lot in this matter: for thy heart is not right in the sight of God. Repent therefore of this thy wickedness, and pray God, if perhaps the thought of thine heart may be forgiven thee. For I perceive that thou art in the gall of bitterness, and in the bond of iniquity." The last is a terribly strong statement, by an apostle, claiming distinct spiritual insight, that Simon is not a Christian.
The language of v17, "Then laid they their hands on them, and they received the Holy Ghost," is no incontrovertible sign that Simon either experienced a saving work of grace by the Spirit, or even participated directly in the "common operations" of the Spirit. He is distinguished in the following v18 by his quest to purchase the apostolic authority of imparting the gifts of the Spirit. In other words, he views the divine mysteries as sorcery--not exactly a Christian attitude--and he thinks they are accessible to those who have gained initiation, so as to have power over others.
Simon's "belief" is no better than the "belief" of many of those who followed Christ for a while, and then abandoned him, Jn2:23-24; 8:30ff; 12:42-43. In other words, people are sometimes described in Scripture according to the profession they make, as believers--even if that belief is later demonstrated to be temporary and ultimately false, cf. 1Jn.2:19.