What is the way to gain deep understanding of the religion, in light of the multiplicity and differences of groups that have corrupted the religion?

The way to gain understanding in the religion is to memorize the Book of Allah Almighty and as much as is facilitated from the Sunnah of His Prophet, peace and blessings be upon him, and to read some concise texts in creed, jurisprudence, principles, and language, and to strive to obtain their explanations from trustworthy scholars.

It is recommended to read Al-Aqeedah Al-Wasitiyyah with the explanation of Sheikh Ibn Uthaymeen or Sheikh Al-Fawzan, and Zad Al-Mustaqni' with the explanation of Sheikh Ibn Uthaymeen, or Al-Mulakhkhas Al-Fiqhi by Sheikh Al-Fawzan, and Sharh Al-Usul by Sheikh Ibn Uthaymeen, may Allah have mercy on him, and also his explanation of Al-Ajrumiyyah.

If these fundamentals are acquired, then one should associate with groups that increase knowledge and good deeds, those who rely on the Book of Allah, the Sunnah of His Messenger, peace and blessings be upon him, and the understanding of the Companions, and who do not engage in backbiting or seek out faults.