And who is better in speech than he who [says: "My Lord is Allâh (believes in His Oneness)," and then stands firm (acts upon His Order), and] invites (men) to Allâh's (Islâmic Monotheism), and does righteous deeds, and says: "I am one of the Muslims."
Invite (mankind, O Muhammad ﺻﻠﻰ ﺍﷲ ﻋﻠﻴﻪ ﻭ ﺳﻠﻢ) to the Way of your Lord (i.e. Islâm) with wisdom (i.e. with the Divine Revelation and the Qur'ân) and fair preaching, and argue with them in a way that is better.
Say (O Muhammad ﺻﻠﻰ ﺍﷲ ﻋﻠﻴﻪ ﻭ ﺳﻠﻢ): "This is my way; I invite unto Allâh (i.e. to the Oneness of Allâh Islâmic Monotheism) with sure knowledge, I and whosoever follows me (also must invite others to Allâh i.e. to the Oneness of Allâh Islâmic Monotheism with sure knowledge).
One who guides to something good has a reward similar to that of its doer.
If anyone calls others to follow right guidance, his reward will be equivalent to those who follow him (in righteousness) without their reward being diminished in any respect, and if anyone invites others to follow error, his sin will be equivalent to that of the people who follow him (in sinfulness) without their sins being diminished in any respect
By Allah, if Allah guides aright even one man through you that is better for you than to possess the most valuable of the camels.
And make ready against them all you can of power
in his Sahih Book of Hadith.
There are many Hadiths to this effect. ShaykhulIslam, Ibn Taymiyyah (may Allah be merciful with him) said:
The word "Ilah (god)" is derived from an Arabic root that signifies the one worthy of worship and obedience.
He is worthy of worship due to His Attributes that necessitate that He is to be loved very much and fully submitted to". Ibn Taymiyyah added: "Ilah means that the beloved and worshipped One Whom hearts love very much, submit to, are humble to, fear [His punishment], hope for [His reward], turn back to in hard times, supplicate to in difficulties, rely on in their interests, resort to, become reassured with His remembrance and find peace in His love all these are dedicated to Allah Alone. Therefore, "La ilaha illa Allah (there is no deity but Allah)" is the truest word; its people are the party of Allah and its deniers are His enemies, who deserve His anger and wrath. If a person says this phrase sincerely, all their affairs, states and senses become correct; otherwise, corruption dominates all their knowledge and deeds.
We implore Allah to guide all Muslims, rulers and subjects, to understand Islam, be steadfast on it and advise people for His sake. We implore Allah to save them from committing what is against that. Indeed, Allah is the only One Capable of doing that. May peace and blessings of Allah be upon our Prophet Muhammad, his family and Companions!