Is doubting the validity of the ruling concerning someone who uttered a word of disbelief in jest, or concealed his Islam without a valid excuse, considered an act of disbelief, given that the questioner only recently learned of these two rulings from the Internet?

Whoever utters a word of disbelief jokingly or playfully disbelieves, because joking about disbelief is disbelief by consensus, whether it is said mockingly, out of stubbornness, or as a belief.

As for concealing Islam, if a person performs the rituals secretly, he does not disbelieve. However, he may be disobedient if he neglects congregational prayer or openly commits major sins. If he does not perform any of the rituals, he cannot be truly considered a believer.

A Muslim's ignorance of this ruling is not disbelief, because it is not from what is necessarily known of the religion. What makes one disbelieve by denying it is what is necessarily known of the religion, which is known by both the elite and the common people, such as prayer and fasting.