There were three main groups of early converts to Islam: younger brothers and sons of great merchants, people who had fallen out of the first rank in their tribe or failed to attain it, and the weak, mostly unprotected, foreigners. Abu Bakr, who used to purchase slaves to set them free in accordance with Muhammad's principle of equality, attracted a large number of converts. Nevertheless, the number of these early converts remained small, and Muhammad concentrated on quietly building a small, but spiritually strong, community. Around 613, the Quran commanded Muhammad to "admonish your nearest kinsmen," initiating the phase of public preaching. One day, Muhammad climed the As Safa mountain, and called out the tribal chiefs. After receiving assurances that the chiefs, who reportedly never heard Muhammad tell lies, would believe him, he declared the Oneness of God. Later Muhammad organized dinners in which he conveyed and advocated the substance of his message. At these events, Muhammad met fierce opposition from one of his uncles, Abu Lahab, while others such as Hamza ibn Abd al-Muttalib and al-'Abbas accepted him. Several reasons have been given for the resentment of the Quraysh toward Muhammad's message, the most accepted is that the Quraysh also did not accept Muhammad as a prophet since he came from a clan less powerful than his opponents'.