What is the ruling on the claim that calling to tawhid (the oneness of Allah) divides people?

Question:
Some people use a method in da'wah (calling others to Islam) where they call to the virtues of deeds and leave out the call to tawhid (the oneness of Allah), saying that it divides people?

Answer:
No, no, this is a mistake; this is due to his ignorance. If he is calling mushrikin (those who associate partners with Allah), he starts with tawhid (the oneness of Allah). But if he is calling Muslims, he encourages them to remain steadfast upon the truth and to obey Allah and His Messenger (peace and blessings be upon him), and to beware of sins; because they are already monotheists.
But if he is calling the people of shirk (associating partners with Allah), he starts them with the call to tawhid (the oneness of Allah) and abandoning shirk. This is what the Prophet (peace and blessings be upon him) started with, and he ordered the Companions to start with it; because shirk is the greatest of sins. We ask Allah for safety.