how did mohmmed bring mercy t mankind

SAH ALE IMRAN [3:164] 

Certainly did Allah confer [great] favor upon the believers when He sent among them a Messenger from themselves, reciting to them His verses and purifying them and teaching them the Book and wisdom, although they had been before in manifest error.

Excerpted from the Tafseer by Dr. Farhat Hashmi

Prior to this the believing people were openly disobedient and Allah sent Prophet Muhammad (pbuh) as a Rahmat ul Alameen (Mercy for Mankind)

Background:

To bring mercy to people is to improve them, and to pull them out of the dungeon of depression and self-destruction that they reside in. This is due to their ignorance of the Almighty and their preoccupation with the hedonism of the present.

How was Prophet Muhammad pbuh so successful in changing the hearts of diehard pagans thus bringing them into the circle of mercy of Allah?

Some guidelines were discussed in this tafseer:

HOW TO APPROACH PEOPLE TO BRING THEM THE MERCY OF ALLAH:

1. To improve a group of people, one should be familiar with those people, friends with them, knowing their ways and genuine in ones intentions with them.
2. Talk simply and succinctly and do not talk in a manner that it goes over their head. Rasoolallah would speak in the dialects of the people who came to see him. Speaking the language of your guest and friend brings him or her closer to you both in understanding (trust) and in expression of thought. Unless you can trust the person you will not listen to them
3. One has to learn to recite the Quran. (For in the recitation of the Quran lies the power of Allah’s words which cannot be replaced by anything else)
4. Ayahs have two meanings: one is the Ayahs of the Quran i.e. the verses of the Quran but also being able to visualize the signs of Allah thus bringing cognitive understanding of Allah’s signs resulting in understanding Allah’s Munificence.
5. Teach techniques for improvement to change the heart from the inside. (Purification: to be explained later)

End Tafseer.