How can a Muslim remain steadfast in his religion and avoid atheism and agnosticism, given the multitude of sects and the contradiction of fatwas between strictness and leniency?

Ignorance of the branches and issues of religion should not lead to doubt in its fundamentals and principles. There is no connection between an error in ijtihad (independent reasoning) and sinking into the quagmire of atheism; Muslims do not differ on the fundamentals of religion and its basic beliefs. Seek refuge with Allah from Satan, and turn to studying the Quran, Sunnah, and faith, and abandon preoccupation with minor issues before mastering the fundamentals. Do not make anyone an authority over people after the Messenger of Allah, peace and blessings be upon him. To judge Islam, refer to the guidance of the Prophet, peace and blessings be upon him, his Sunnah, and his biography as understood by the Companions. It is recommended to read "Tayseer al-Kareem al-Rahman fi Tafsir Kalam al-Mannan" by Shaykh Abd al-Rahman al-Sa'di, and "And Indeed, You Are of a Great Moral Character" by Shaykh Safiur Rahman al-Mubarakpuri.