Is it necessary, in order to know the truth and abhor falsehood in the most complete manner, for a Muslim to be familiar with all forms of widespread falsehood and religions of disbelief, including those in remote lands, or is it sufficient to know the path of the people of falsehood from the Qur'an and Sunnah?

There is no doubt that knowing the falsity of falsehood increases a believer's certainty in the truth brought by Islam. However, this does not mean that a Muslim should delve into the misguided religions. The Qur'an and Sunnah (definition) have sufficiently clarified truth and falsehood for us. Allah Almighty has clearly explained in His Book the path of believers and evildoers. The Prophet, peace and blessings be upon him, did not command his Companions to teach their children matters of Jahiliyyah (pre-Islamic ignorance), but rather the Book of Allah and the Sunnah of His Messenger. For this reason, you find in books of Tawhid (definition) (monotheism) an explanation of shirk (definition) (polytheism) and its types. The problem and deviation among Muslims is caused by abandoning the Qur'an and Sunnah and neglecting to ponder over them. The details of deviation are needed by some specialized scholars to confront those who propagate falsehood.