Islam literally means to submit and surrender to another's will and in the Quran, it to one who is in total submission to Allah (swt) and is free from any form of shirk and polytheism. It is on these premises that the Quran refers to Prophet Ibrahim (pbuh) as a "Muslim". Since the true acceptable religion to Allah (swt) is Islam[i], all of those who accepted Allah's (swt) religion in their own time [before the religion of Prophet Muhammad (pbuh)] were also Muslim, meaning that they were in total submission to Allah's will. Nevertheless, today those who are considered Muslim are those who follow the religion of the final prophet, Prophet Muhammad (pbuh). That is because they have proven their submission to Him by accepting His final prophet and all the previous ones as well, while followers of other religions today lack this belief, hence their kufr. Of course, the Islam of those Muslims who don’t practice it in their everyday lives is merely a formality and the true Muslim is one who practices it and obeys all of Allah's (swt) commands; both commands that are personal and have nothing to do with others, such as prayer and fasting, and commands He has issued regarding others and their rights.
The term "Islam" literally means total submission and surrender, and to have absolute obedience without question.
The "Muslim" is one who is in total submission to God's (swt) will and command; accepting everything He has in store for him and obeying all of His commands.[1]
These were the definitions for the terms above. "Islam" also refers to the religion of the seal of prophets, Prophet Muhammad (pbuh).[2] The reason why this religion has been called "Islam" is that it has to do with total submission and obedience of its laws without any objection.[3]
In the Quran, those who follow the religion of Prophet Muhammad (pbuh) aren’t the only ones called Muslim; all of those who are in total submission to Allah's (swt) command and are free of all shirk and polytheism are considered Muslim, that is why even Prophet Ibrahim (pbuh) has also been called Muslim therein as well.[4]
When Allah (swt) says: "Say," I have been commanded to be the first of Muslims"[5], what is meant by being the first Muslim is being the first to submit to Allah (swt) amongst his [Prophet Muhammad's] tribe and nation, not amongst all prophets, because clearly, there were prophets before him who had submitted to Allah (swt) as well.
Anyway, what is concluded from verse 85 of surah Al-Imran (that says no religion other than Islam is accepted by Allah (swt)), and from verse 3 of surah Maidah (that says "Today I have perfected your religion for you, and I have completed My blessing upon you, and I have approved Islam as your religion) is that today the only people who are considered Muslim are those who have embraced the religion of Islam, because by doing so, they have acknowledged all previous prophets as well and thus have submitted to Allah (swt). Conversely, followers of other religions today can't be considered Muslim, meaning that they aren’t in total submission to Allah (swt), because they haven’t accepted Prophet Muhammad's (pbuh) prophethood, thus straying from Allah's (swt) command. Of course it should be noted that the Islam of those who have accepted Prophet Muhammad's prophethood but don’t practice their beliefs devotedly is merely a formality and isn’t true Islam; true Islam is to testify to God's oneness and the prophethood of His final prophet, and to obey Him in His commands.
The true Muslim is one who observes others' rights and the Islamic laws pertaining to them, and also observes Islamic laws that are personal and have nothing to do with others, such as prayer and fasting.[6]
[1] Muhammad Husein Tabatabai, Al-Mizan (translation), vol.1, pg. 454.
[2] Al-Wujuh wal-Nadha'ir, vol.1, pg.248.
[3] Mabadil-Islam, pg.7.
[4] Al-Imran:67 " مَا کاَنَ إِبْرَاهِیمُ یهَُودِیًّا وَ لَا نَصْرَانِیًّا وَ لَاکِن کاَنَ حَنِیفًا مُّسْلِمًا وَ مَا کاَنَ مِنَ الْمُشْرِکِین" (Abraham was neither a Jew nor a Christian. Rather he was a Hanif, a Muslim, and he was not one of the polytheists)
[5] An'am:14 " قُلْ إِنىِّ أُمِرْتُ أَنْ أَکُونَ أَوَّلَ مَنْ أَسْلَمَ وَ لَا تَکُونَنَّ مِنَ الْمُشْرِکِین"
[6] With help from Question 664 (website:), Index: Quran and the meaning of Islam and Muslim.