What are the matters with which preachers should begin in Islamic da‘wah?

It is prescribed to begin with creed, as the Prophet, peace and blessings be upon him, and the Messengers did, based on the hadith of Mu'adh. If those invited are disbelievers, begin by inviting them to the two testimonies (Shahadatayn), then prayer (Salah), then charity (Zakah). But if they are Muslims, clarify for them what they are ignorant of regarding the rulings of their religion, and attend to what is most important, then what is important.