The literary meaning of Islam is obedience and submission and terminologically it means acceptance of the religion of Islam. Prophet Muhammad (s) propagated the religion of Islam and his followers are called as Muslims, because they have surrendered themselves to Allah and His heavenly laws.
The Holy Quran says:
وَمَنْ أَحْسَنُ دِيناً مِمَّنْ أَسْلَمَ وَجْهَهُ لِلَّهِ وَهُوَ مُحْسِنٌ وَاتَّبَعَ مِلَّةَ إِبْرَاهِيْمَ حَنِيفاً وَاتَّخَذَ اللَّهُ إِبْرَاهِيمَ خَلِيلاً
“And who has a better religion than he who submits himself entirely to Allah? And he is the doer of good (to others) and follows the faith of Ibrahim, the upright one, and Allah took Ibrahim as a friend.” (4:125)
In Quran, it is clearly mentioned that the religion of Ibrahim (a) was also in the meaning of Islam. He was a Muslim and requested Allah for his descendants and community to be Muslim. In the same way, it is mentioned in Quran:
مَا كَانَ إِبْرَهِيمُ يَهُودِيّاً وَلَا نَصْرَانِيّاً وَلَكِنْ كَانَ حَنِيفاً مُّسْلِماً وَ مَا كَانَ مِنَ الْمُشْرِكِينَ
“Ibrahim was not a Jew or a Christian but he was (an) upright (man), a Muslim, and he was not one of the polytheists.” (3:67)
And He says:
رَبَّنَا وَاجْعَلْنَا مُسْلِمَيْنِ لَكَ وَمِنْ ذُرِّيَّتِنَآ أُمَّةً مُّسْلِمَةً لَّكَ وَأَرِنَا مَنَاسِكَنَا وَتُبْ عَلَيْنَآ إِنَّكَ أَنْتَ التَّوَّابُ الرَّحِيمُ
“Our Lord, and make us both submissive to Thee and (raise) from our offspring a nation submitting to Thee, and show us our ways of devotion and turn to us (mercifully), surely Thou art the Oft-returning (to mercy), the Merciful.” (2:128)
In the same way, He says:
وَ جَهِدُواْ فِى اللَّهِ حَقَّ جِهَادِهِ هُوَ اجْتَبَكُمْ وَ مَا جَعَلَ عَلَيْكُم فِى الدِّينِ مِنْ حَرَجٍ مِلَّةَ أَبِيكُمْ اءِبْرَاهِيمَ هُوَ سَمَّكُمُ الْمُسْلِمِينَ مِن قَبْلُ
“And strive hard in (the way of) Allah, (such) a striving as is due to Him; He has chosen you and has not laid upon you any hardship in religion; the faith of your father Ibrahim; He named you Muslims before…” (22:78)
And also, He says:
إِنَّ الْدِّينَ عِنْدَ اللَّهِ الْإِسْلَمُ وَمَا اخْتَلَفَ الَّذِينَ أُوتُواْ الْكِتَبَ إِلَّا مِنْ بَعْدِ مَا جَآءَهُمْ الْعِلْمُ بَغْيَاً بَيْنَهُمْ وَمَنْ يَكْفُرْ بَِايَتِ اللَّهِ فَإِنَّ اللَّهَ سَرِيعُ الْحِسَابِ
“Surely the (true) religion with Allah is Islam, and those to whom the Book had been given did not show opposition, but after knowledge had come to them, out of envy among themselves; and whoever disbelieves in the communications of Allah then surely Allah is quick in reckoning.” (3:19)
In another place, He says:
وَمَن يَبْتَغِ غَيْرَ الْإِسْلمِ دِيناً فَلَنْ يُقْبَلَ مِنْهُ وَهُوَ فِى الْأَخِرَةِ مِنَ الْخَسِرِينَ
“And whoever desires a religion other than Islam, it shall not be accepted from him, and in the hereafter he shall be one of the losers.” (3:85)
One who confesses to the three fundamentals would be considered a Muslim: Monotheism, resurrection and prophethood of Prophet Muhammad (s). Acceptance of these three fundamentals makes him eligible for legacies of Islam like purification, justification of marriage with Muslims, inheritance and security of life and property.
Qasim Sairafi says: Imam Ja’far Sadiq (a) said: By Islam, life of man remains sacred and secure, his trusts are restored and marriage becomes lawful with him, but the (divine) reward depends on faith.
However, every Islam does not become eligible for success in the hereafter; on the contrary these effects are only related to the Islam, which springs from the inner conscience of man and which is accompanied with acting on the religious duties. From this aspect in traditions performance of obligatory acts is introduced as a pillar of Islam. Abu Hamza has narrated from Imam Muhammad Baqir (a) that he said: Islam is based on five pillars: Prayer, Zakat, Fast, Hajj and Wilayat. However nothing is emphasized as much as Wilayat.
 Al-Kafi, Vol. 2, Pg. 24
 Al-Kafi, Vol. 2, Pg. 18