Imam Ja'far al-Sadiq (a) narrates:
Once the Messenger of Allah (s) sent Imam Ali (a) to Yemen to guide people to Islam, teach them Islamic laws and regulations, and to spread Islam among the people. He said to Imam Ali (a):
“Do not declare war on any person unless you have first invited him to Islam.”
This meant that he should begin the mission by spreading Islam and explaining its principles to non-Muslims through clear proofs and reason. This is so because Islam is the religion of reason and knowledge. If any enter the faith, they become your brothers in religion. If they refuse Islam then he should fight them for persisting in their infidelity and unbelief, and wanting to defend oppression and corruption.
The Messenger of Allah (s) said to Imam Ali (a), after explaining to him how to spread Islam and convey its message:
“...if Allah guides only one person through you, it will be better for you than all that the sun has risen or set over.”1
By this noble hadith, the Messenger of Allah (s) explains the significance of Islamic call Da'wah and its great importance to Allah, the Almighty. He also points out the greatness of those who invite or call others to Islam. These missionaries undertake the task of spreading Islam and inviting non-Muslims to enter into the faith, thus saving them from disbelief, oppression and corruption. They also undertake the duty of guiding every deviated one who has gone astray from the right path, by teaching Islamic laws, education and thoughts. They also explain the right Islamic concepts.
Surely, whoever guides a deviator, whether non-believer in Islam or a Muslim who commits evil deeds but misunderstands Islam, gains more to his own benefit than owning the whole world because the possessions of the world perish while the reward of guiding people will remain permanent with Allah and will be a cause of Allah's pleasure. The believer who guides people will enter Paradise. Guiding others will gain the reward of Paradise and Allah's pleasure, while possessing the whole world with its property and wealth will gain nothing in the Hereafter.
It is obligatory for every Muslim to be a caller to Islam and to spread the word of Islam in every corner of the world. Today there are more than one billion Muslims! Believers, through calling others, can convey the Islamic religion to mankind and spread its message to different parts of the world. Such work guides and reforms man. Without calling people to Islam no one would be guided and the true religion can not spread to others.
Muslims themselves should learn about Islam and act according to its laws and principles, inviting others to believe in Islam and act accordingly. Indeed guiding one person is better than whatever man possesses in this world because this guidance is a step towards saving people and reforming them. The meaning of guidance and Islamic Call Da'wah is to spread Islam among people and to teach them Islamic laws, principles and ideologies, as well as how to act and believe according to these laws.