All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The best way of calling non-Muslims to Islam is to inform them of the demands of this religion, the most important of which is the belief in the Oneness of Allaah which prohibits a person from worshipping other than Him. On one hand this requires, of course, religious knowledge.
On the other hand, one has to show them that other religions have many deviations and contradictions, which implies that they are false and that one cannot affiliate with them as they do not lead to salvation. Evidently this requires a person to have a wide knowledge about these religions and must have read books in comparative religions.
In addition to the above, one must invite them with wisdom and fair preaching and be patient about any harm that could occur from them.
A Muslim who invites them has to be an example to them with regard to his relations with others and his moral conduct.
The Prophet, sallallaahu alayhi wa salam, used to call non-Muslims to utter the two testimonies of faith and to embrace Islam, immediately after presenting the truth to them. This took place many times, among which is his visit to the young Jew and his inviting him to Islam saying to him: Say: ''Laa ilaaha illallaah'' (none is worthy of being worshipped but Allaah). The story is reported in Al-Musnad of Imaam Ahmed.
Allaah knows best.