Why Didn't Simon the Sorcerer's Belief Save Him?

If man's belief is what saves him, then what about Simon the sorcerer? After all he believed and was baptized:

Acts 8:12 But when they believed Philip preaching the things concerning the kingdom of God, and the name of Jesus Christ, they were baptized, both men and women.

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.

Yet, despite the fact that he believed. And his belief led him to even be baptized. We know absolutely that he was NOT saved. We know he was not saved from the Word of God:

Acts 8:18 And when Simon saw that through laying on of the apostles' hands the Holy Ghost was given, he offered them money,

19 Saying, Give me also this power, that on whomsoever I lay hands, he may receive the Holy Ghost.

20 But Peter said unto him, Thy money perish with thee, because thou hast thought that the gift of God may be purchased with money.

21 Thou hast neither part nor lot in this matter: for thy heart is not right in the sight of God.

22 Repent therefore of this thy wickedness, and pray God, if perhaps the thought of thine heart may be forgiven thee.

23 For I perceive that thou art in the gall of bitterness, and in the bond of iniquity.

Notice the language: "thy heart is not right in the sight of God". Oh, but his mouth was right. He believed. His actions were right. He was baptized. None of that matters though if the heart remains unchanged. And tragically, that's how it is with hundreds and hundreds of millions of professed Christians today. They believe with their mouths and take outwards actions (like baptism) – but the heart remains as stone.