Perhaps the most oft-quoted reference with regards to priorities and gradation in da`wah is the famous hadeeth reported by Ibn `Abbaaas radiyAllahu `anhu: The Prophet sallAllahu `alaihi wa sallam sent Mu`aadh to Yemen and said, "Invite the people to testify that Laa ilaaha ill Allah [none has the right to be worshipped but Allah] and that I am the Messenger of Allah. And if they obey you in that, then inform them that Allah has enjoined upon them five daily prayers in every day and night, and if they obey you in that, then inform them that Allah has made it obligatory for them to pay the Zakaat from their properties and it is to be taken from the wealthy among them and given to the poor among them."7
Another narration states: "You are going to a nation from from the people of the Book, so the first you should invite them to is the assertation of the oneness of Allah..."8
Muhammad (s) also said: "I have been ordered to fight against the people until they testify that Laa ilaaha ill Allah wa anna Muhammad ar-Rasool Allah, and offer prayers perfectly, and give Zakaat so if they perform all that, then they save their lives and properties from me except for Islamic laws, and then their reckoning (accounts) will be with Allah."9
"I have been sent before the Hour so that Allah alone should be worshipped without any partner for Him, and my provision has been placed beneath the shade of my spear, and subservience and humiliation have been placed upon those who disobey my orders, and whoever imitates a people then he is one of them."10
It was for this reason, that the Prophet(s) stayed in Makkah for thirteen remarkable years, calling people to the correct understanding of Oneness of Allah (swt), and making worship solely for Him.