What must Islamic preachers do first?

The Shaykh : It is also obligatory for Islamic preachers to begin with the testimony that there is no god but Allah and that Muhammad is the Messenger of Allah, by explaining and elaborating on it, not just by reciting the words. Therefore, it is not permissible for us to say that we leave aside jurisprudence and scholarly disagreements, because that means leaving the call to the testimony that there is no god but Allah and that Muhammad is the Messenger of Allah. What is the fourth point?
Ali Hasan : The Islamic groups

The Shaykh : They also do not criticize the Islamic groups. What is this? I am a Salafi and you are a Khalafi, so do not criticize me. Why? Because I am right or because I am wrong? No, this creates division. What is the benefit of your call if you leave me in my misguidance, and what is the benefit of my call if I leave you in your misguidance? And so on. We must speak the word of truth. They have not read in the book of the Companions that the Prophet (peace and blessings be upon him) advised Abu Dharr with several pieces of advice, one of which was: "Let no one deter you from speaking for the sake of Allah." What is this? This is a section, this is one of the advices of the Messenger (peace be upon him) to Abu Dharr. Therefore, we must learn and act upon what we learn. Turning away from these four points, or clinging to them, means turning away from adhering to the Islam brought by him (peace and blessings be upon him).