The Prophet (sal Allahu alaihi wa sallam) said to Muadth bin Jabal (radi Allahu anhu) “You are going to a people from the People of the Book. Let the first thing that you call them to be the worship of Allah. If they acknowledge Allah, then inform them that Allah has obligated upon them five prayers during their days and nights.”
[Sahih Al-Bukhari and Muslim]
In the above Hadith the Prophet (sal Allahu alaihi wa sallam) instructed Muadth bin Jabal (radi Allahu anhu) to call the people of Yemen to Tawheed (i.e. the worship of Allah alone) first and foremost and then when they accept and acknowledge then call them to that which is next in order of importance which is the five daily prayers etc.
Tawheed is the first thing that a Muslim should learn and then Call to, as Allah says (interpretation of the meaning): “So know that there is no God except Allah, and ask forgiveness for your sin and for the believing men and the believing women” [Muhammad, 47:19]
Jundub Ibn Abdullah al-Bajaly (may Allah be pleased with him) said, “We learned Eeman (faith) and then we learned the Qur’an and it increased our Eeman.”
And Allah knows best!