Is a Muslim sinful if he stops thinking about non-forbidden religious matters?

You do not sin by diverting your thought from what was mentioned. Thinking about matters beneficial for one's religion and worldly life is a good thing that increases faith. Strive to guard your thoughts from bad ideas, and occupy yourself with thinking about what benefits you in matters of your religion and worldly life. You should recall these thoughts if your mind wanders to something else, unless you intend to engage in something more important.