The Prophet (saw) giving dawah to a Christian.
Here is a small conversation that took place between our noble messenger of Allah (saw) & Adi Ibn Hatim.
(Translated and adapted from Sheikh Muhammad Ash-Shanqiti’s Manhaj Ar-Rasul Fi Da`wat Ahl Al-Kitab)
They have taken their scholars and monks as lords besides Allah…
Imam Ahmad narrated on the authority of `Adi, “I came to the house of the Prophet (peace be upon him) and he said,: “O `Adi, accept Islam and you will be safe!” I replied, “I already have a religion.”
The Prophet said, “I know more about religion than you do.” I said, “You know more about my religion than I do?” The Prophet said, “Yes, are not you from the Rukusiyyah (a sub-sect within Christianity and Sabeans)? And when you go to war, do you not share one quarter of your people’s booties?” I said, “Yes.” The Prophet (peace be upon him) said, “I know what is preventing you from accepting Islam.
You think that the only people to follow this religion are the oppressed people who have no strength: the Arabs have discarded them. O ‘Adi! Have you heard of Al-Hira (a city in ‘Iraq)?” I said, “I have not seen it, but I have heard of it?” After that, the Prophet (peace be upon him) said, “I swear by the One Who has my soul in His Hand, Allah will complete this affair, until a woman travels from Al-Hira to make circumambulation around the Ka`bah, without fearing anyone, and the treasures of Kisra ibn Hurmuz will be seized.” I said, “The treasures of Kisra ibn Hurmuz?” The Prophet said, “Yes, Kisra ibn Hurmuz, and money will be spent until none would accept it.” (Ahmad)