Sheikh Hamed Al-`Ali, instructor of Islamic Heritage at the Faculty of Education, Kuwait and Imam of Dahiat As-Sabahiyya Mosque, states that: Kufr and Shirk sometimes have the same meaning and sometimes are different. If they are mentioned in the same context, then each one of them has a separate meaning, i.e. Kufr would mean to deny or disbelieve in something that entails leaving Islam such as Salah (Prayer), Zakah (obligatory charity) and everything whose obligation has been established by scholarly consensus (Ijma`), or denying things that entail apostasy from Islam such as worshipping another god other than Allah. Mainly Kufr means denial or rejection of Allah’s Divinity.
Shirk – if mentioned with Kufr in the same context –means associating other partners with Allah, the Almighty. However, if each one of them is mentioned in different context, they would have the same meanings and implications. Thus a Kafir can be called a Mushrik since he or she has taken another god other than Allah. Allah the Almighty says in the Qur’an: (Hast thou seen him who maketh his desire his god.) (Al-Jathiyah: 23) By the same token a Mushrik may be called Kafir since he or she has denied with his saying, action, or thought the Oneness of Allah. The same applies to Islam and Iman; both bear the same meaning even if they are mentioned in different contexts. An example of this is the hadith of the Prophet: “A Muslim is he from whose hand and tongue the Muslims are safe.” (Reported by Muslim). The “Muslim” here means Mu’min (a believer). And the hadith: “Iman (Faith) has over seventy branches, the most excellent of which is to bear witness that there is no god but Allah, and the lowest of which is the removal of hazardous substances from road.” (Reported by Muslim). Iman here means Islam.
But if they are mentioned in the same context, then each of them would have a different meaning. An example of this is the hadith reported by Ibn `Umar that Jabril (peace and blessings be upon him) came to the Prophet (peace and blessings be upon him) and said: “Muhammad, tell me about Islam.’ The Messenger of Allah (peace and blessings be upon him) said: `Islam is to testify there is no god but Allah and that Muhammad is the Messenger of Allah, and to perform Prayer, give Zakah, fast in Ramadan, and perform the pilgrimage to the House (in Makkah) if you can afford the journey.’ He (Jabril) said: `You have spoken the truth,’ and we were surprised that he was doing both the questioning and approving the answer. Then he said: `Tell me about true faith (Iman), ‘ and the Prophet (peace and blessings be upon him) answered: `It is to believe in Allah, His angels, His inspired Books, His messengers, the Last Day, and in destiny, its good and evil.”
In this hadith, the word Iman means the belief that is reposed in the heart, while the word Islam implies the complete surrender to Allah, reflected through the religious observances and one’s actions. That is why the Prophet (peace and blessings be upon him) defined Islam as “to testify that there is no god but Allah and that Muhammad is the Messenger of Allah, and to perform Prayer, give Zakah, fast in Ramadan, and perform the pilgrimage to the House (in Makkah) if you can afford the journey.’ And he (peace and blessings be upon him) defined Iman as “to believe in Allah, His angels, His inspired Books, His messengers, the Last Day, and in destiny, its good and evil.” However, Iman is not valid unless it’s reflected through action, but it is not a condition that one has to be fully obedient and surrendering to Allah in everything so that he or she can be called a believer. That is why we consider the people who commit major sins as sinful Muslims. Likewise, Islam is not valid unless one believes in the religion and has conviction in his/her heart, and the same applies to Iman.