Invite to the way of your Lord with wisdom and fair preaching, and argue with them with that which is best. Truly, your Lord best knows who has strayed from His path, and He best knows those who are guided.

-- An-Nahl 16:125

Allah commands His Messenger Muhammad to invite the people to Allah with Hikmah (wisdom). Ibn Jarir said: "That is what was revealed to him from the Book and the Sunnah."

His command of "fair preaching" means with exhortation and stories of the events that happened to people mentioned in the Qur'an, (At-Tabari 17:321) which he is to tell them about in order to warn them of the punishment of Allah.

The command to "argue with them with that which is best" means that if any of them want to debate and argue, then let that be in the best manner, with kindness, gentleness, and good speech, as Allah says elsewhere:

And do not argue with the People of the Book, unless it be with that which is best, except for those who purposefully do wrong.

--29:46

Allah commanded him to speak gently, as He commanded Musa (Moses) and Harun (Aaron) to do when he sent them to Pharaoh, as He said: "And speak to him mildly, perhaps he may accept admonition or fear" (Allah) (20:44).

"Truly, your Lord best knows who has strayed from His path" means that Allah already knows who is doomed (destined for Hell) and who is blessed (destined for Paradise). This has already been written with Him and the matter is finished, so call them to Allah, but do not exhaust yourself with regret over those who go astray, for it is not your task to guide them. You are just a warner and all you have to do is convey the Message; it is He Who will bring them to account.

Tafsir Ibn Kathir Vol. 5

"Let The Sunnah Go Forth And Don't Stop The Sunnah"

The Protectors of the Sunnah (POTS) is an Islamic networking group established to provide a forum for Muslims who subscribe to the belief that we live our lives according to the Qur'an and authentic traditions (the Sunnah) of the Prophet Muhammad (pbuh) as understood by his close Companions. POTS is committed to the revival of Islam in its purest form.