Citing the hadith of Mu'adh (may Allah be pleased with him) regarding the method of da'wah.

The Shaykh : When the Messenger (peace and blessings be upon him) sent Mu'adh to Yemen, he said to him: . I am telling you: No, the Messenger says: Let the first thing you call people to be the testimony that there is no god but Allah. Whether the people are disbelievers or Muslims; why? Here, knowledge now shows you how the matter differs. With disbelievers, one restricts oneself and is content with them until they become Muslims by saying: "There is no god but Allah, Muhammad is the Messenger of Allah," until they become Muslims. However, they do not thereby become believers; if the disbeliever says: "There is no god but Allah, Muhammad is the Messenger of Allah," he becomes a Muslim, he has what we have and he is bound by what binds us, but he does not thereby become a believer, meaning one who is saved with Allah (Blessed and Exalted is He). How? And why?
The answer is that the two testimonies are a declaration of Islam, but this does not mean that he understood Islam, which begins with faith. This takes time. With time, it will become clear to the Muslim community and the Muslim ruler whether this disbeliever who said: "I testify that there is no god but Allah," said it out of hypocrisy, said it out of fear, out of fear of Islam, out of fear of the jizyah, or out of sincerity. With time, it will become clear. So if you are calling a Muslim, the first thing you call him to is the testimony that there is no god but Allah. But here the matter differs. This person is a witness to Allah, he prays, and he testifies in prayer and outside of prayer. But here, your function now is to explain to him the testimony that there is no god but Allah. The disbeliever does not need explanation; because this, if we may use the expression, we consider it like bait to lure him to Islam. The key is that he testifies that there is no god but Allah. But if he is to be saved on the Day of meeting Allah (Mighty and Majestic) and be among the people of Paradise, this key must, as some of the Salaf said: Every key must have teeth; if it has no teeth, it does not open. So this key, "There is no god but Allah," must have teeth. What are its teeth? The completion of the pillars of faith and Islam, such as prayer, and so on.
But today, Muslims, especially the non-Arabs, even the Arabs...