"As to those who reject Faith, It is the same to them Whether thou warn them Or do not warn them; They will not believe." The Holy Quran, 02:06 Al Baqarah.
Abdullah Yusuf Ali's commentary: Kufr, Kafara, Kafir, and derivative forms of the word, imply a deliberate rejection of Faith as opposed to a mistaken idea of Allah or faith, which is not consistent with an ernest desire to see the truth. Where there is sch desire, the Grace and Mercy of Allah gives guidance. But that guidance is not efficacious when it is deliberately rejected, and the possibility of rejection follows from the grant of free will. The consequence of the rejection is that spiritual faculties become dead or impervious to better influence.
Be careful who you call Kafir because only Allah knows who is truly a rejector and who is merely a misguided person. It is not up to us to "sentence" a person to the Hellfire.
We must always maintain courtesy, diplomacy and a never-dying zeal to convey the message to others no matter how discouraging or useless it may seem - don't give up on a non-muslim because most people are good people who simply need a break from the anti-Islamic propaganda around them.
The Fastest Way to Leave Islam
It is always safest to assume that a non-muslim has been misguided, misinformed and fed lies and misconceptions about Islam and Muslims - don't jump to calling him a Kafir. Remove the name-calling from your articles and websites - wouldn't you rather be safe than sorry - name-calling, especially wrongly branding someone as 'evil', 'kafr' and 'manifestation of satan' - GHEEBOT - could entail dire consequences for us on Judgement Day.
Perhaps there is no faster way to leave Islam than by calling other Muslims "kafir" or "mushrik" without discrimination.
About calling other Muslims "kafir," we read the following hadith: It is reported on the authority of Ibn `Umar that the Prophet (may peace and blessings be upon him) said: Any person who calls his brother: O Unbeliever! (then the truth of this label) would return to one of them. If it is true, (then it is) as he asserted, (but if it is not true), then it returns to him (and thus the person who made the accusation is an Unbeliever). [Muslim]
Therefore, if you call other Muslims "kafir" or "mushrik" without discrimination, you could find that you have left Islam, according to the words of the Prophet (s.a.w.), in less than a second.
Waste no time debating what a good Muslim should be. Be one!
Guiding a Non-Muslim to Islam only possible by the Grace of God. Our job is to help others understand Islam - that is our basic duty. And only then inshallah, will non-muslims move in the direction of becoming one of us. We must remain positive, open-minded and generous in our efforts to convey the true message.
You cannot encourage a non-muslim to be understanding and open-minded if you yourself are hot-tempered and hurling accusations at him/her. No non-muslim is going to respect you and it will certainly reflect badly on Islam if a Muslim, a representative of Islam, is narrow-minded.
I pray to Allah that we develop patience and sincerity in our efforts to bring about more tolerance in this world. Remember....every human being, regardless of whether he/she is born to Muslim or Non-Muslim parents, is born innocent and pure...it is society that influences him/her to stray from Islam and just as a person can stray, he/she can revert back and YOU can help this process.
We have a very important and sensitive role to play. We are the trusted Ummah of the Rasool, a servant of God...it has been made our duty and our obligation to pass on the pure, unadulterated truth - make honest and full use of this honourable task.