How can a common person advise a warring disbeliever to cease his corruption and embrace Islam, and what are the priorities for such advice? Must he study Sharia sciences to do that, or is it permissible for him to advise only with what he knows?

The priority in inviting non-Muslims is to call them to the Two Testimonies (Shahadatayn) and to enter Islam, as stated in the Hadith of Mu'adh. Knowledge is a prerequisite for inviting to Allah, so a Muslim should invite others to what he knows of the religion, even if it is little. It is not permissible for him to invite to something of which he has no knowledge. A Muslim must convey what knowledge he possesses, whether it is little or much, but it is forbidden for him to speak about what he does not know or to delve into it. It is not a condition for the inviter (da'i) to be a profound scholar; rather, it is sufficient for him to be knowledgeable about the specific issue he is inviting to. As for discussing the specious arguments of disbelievers, the inviter must possess sufficient religious knowledge to refute these arguments and establish the proof.