In short, if Islam is all we need to know, it begs the question, why does the Qur'an talk about Christians, Jews, Polytheists and even Ancient Egyptians.
The wisdom behind the Qur'an and ahadith telling Muslims about these parts of history outside Prophetic period of Muhammed(saw) and Muslim history of his Ummah is quite straightforward.
For Muslims to be witnesses to the truth, to mankind.
The purpose of studying history is to learn from the mistakes of our predecessors and learn what made them successful. The revelation is replete with histories of people who denied, defied, deride, or conversely, even deified some Prophets. These are all important lessons that the author of the Qur'an (SWT) judged that we needed to know, to avoid making the same mistakes.
It is difficult to guide someone who is lost if you don't know where they are. To orient ourselves politically in our collective work we must understand the peoples of other nations, tribes and their cultures and belief systems?
Because the Muslims understood that the just Christian leadership of Abyssinia were unlike the Christian leadership of the (Eastern) Roman Empire, they treated them differently seeking refuge with them from persecution. Without knowing about the beliefs and culture of the Abyssinians, they wouldn't have been able to deftly appeal to their values of justice in front of the Pagan Arab emissaries - who demanded the Muslim refugees be turned over to them for "punishment".
The Christian hegemony of the (Eastern) Roman Empire no longer exists. However, the inheritors of the (Western) Roman Empire still exist and they are the descendants (or invading pagan Germanic inheritors) of the Christians at the time of Muhammed (SAW). They've change quite radically since then and studying Christianity or Judaism alone, simply won't have the same usefulness it once did 1,300 years ago, when Muslim scholars began writing the first detailed polemic against Christians, or Muslim Caliphs organising the first diplomatic, political (and sometimes military) engagements with the "franji" (Frankish peoples, i.e. Western Europeans).
The study of Western civilisation is no more useless, or fraught with dangers of doubt, as it was when the Qur'an and Sunnah explained Christianity, Judaism and the realities of Polytheism to the first Muslims.
Therefore the benefit in studying non-Islamic beliefs and religions (as well as different civilisations) is:
1) Knowing someone's belief allows you to call them to the Truth of Islam.
2) Learning about past and present civilisations and communities of belief, allows us to learn lessons about how not to replicate their mistakes, misconceptions and false assumptions about the universality of their ideas and world-views. This allows us to identify where we have inadvertently adopted these same ideas and allow us to recognise this. This is especially necessary when they have had profound influence on Muslim culture and thinking in recent history.
3) Knowing about a people, their history, origins, culture and worldview allows you to formulate strategies to politically engage with them, whether collaborating in good works with them, or opposing injustice amongst/from some of them.
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