The surah takes its name Al-Mu'minun from the first verse.
Period of Revelation
Both its style and theme indicate that it was revealed during the middle stage of Prophethood at Mecca. Reading between the lines one feels that a bitter conflict had begun between the Holy Prophet and the disbelievers of Mecca though the persecution by them had not yet become tyrannical. It appears that the surah was sent down during the climax of the "Famine" in Mecca (vv. 75-76) which according to authentic traditions occurred during the middle stage of Prophethood. Moreover according to a tradition related by 'Urwah ibn Zubair
Umar who had embraced Islam by that time said This Surah was revealed in my presence and I myself observed the state of the Holy Prophet during its revelation.
The central theme of the sura is to invite the people to accept and follow the Message of the Holy Prophet and the whole Sura revolves round this theme.