What is the ruling on one who does not know Tawhid, and are there Muslims who do not know Tawhid or that Muhammad is the Messenger of Allah?

If there are people who know nothing of Islam but the phrase "Lā ilāha illā Allāh" (There is no god but Allah) during a time when Islam has faded, they are Muslims and this phrase will benefit them, as mentioned in the hadith of Hudhayfah ibn al-Yamān. As for those who heard of Prophet Muhammad (peace be upon him) but did not believe in him, their claim of monotheism will not benefit them; rather, they are disbelievers, and it is incumbent upon them to believe in his message. Everyone among the polytheists who hears mention of the Prophet (peace be upon him) is obligated to inquire about his status and believe in him. And whoever has not heard mention of him (peace be upon him), if they are monotheists, they are believers and among the people of Paradise. If they are not monotheists, they will be punished unless they enter the Hellfire prepared for them on the Day of Judgment. There is no punishment for a disbeliever until the warning of the Messenger reaches them. And whoever has heard mention of the Prophet but cannot find anyone in their land to inform them about him, it is obligatory for them to emigrate to learn the truth.