Islam, an Arabic word which, since Mohammed's time, has acquired a religious and technical significance denoting the religion of Mohammed and of the Koran, just as Christianity denotes that of Jesus and of the Gospels, or Judaism that of Moses, the Prophets, and of the Old Testament.
Grammatically, the word Islam is the infinitive of the so-called fourth verbal form of the regular intransitive stem salima, "to be safe", "to be secure", etc. In its second verbal form (sallama) it means "to make some one safe" and "to free", "to make secure", etc. In its third form (salama), it signifies "to make peace", or "to become at peace", i.e. to be reconciled". In its fourth form (aslama), the infinitive of which is islam, it acquires the sense of "to resign", "to submit oneself" or "to surrender". Hence Islam, in its ethico-religious significance, means the "entire surrender of the will to God", and its professors are called Muslimun (sing. Muslim), which is the participial form, that is "those who have surrendered themselves", or "believers", as opposed to the "rejectors" of the Divine message, who are called Kafirs, Mushriks (that is those who associate various gods with the Deity), or pagans.
Historically, of course, to become a Muslim was to become a follower of Mohammed and of his religion; and it is very doubtful whether the earliest Muslims or followers of Mohammed, had any clear notion of the ethico-religious significance of the term, although its later theological development is entirely consistent and logical. According to the Shafiites (one of the four great Mohammedan schools of theology), Islam, as a principle of the law of God, is "the manifesting of humility or submission, and outward conforming with the law of God, and the taking upon oneself to do or to say as the Prophet has done or said"; and if this outward manifestation of religion is coupled with "a firm and internal belief of the heart", i.e. faith, then it is called Iman. Hence the Mohammedan theological axiom "Islam is with the tongue, and Iman is with the heart." According to the Hanafites (another of the four above-mentioned schools), however, no distinction is to be made between the two terms, as Iman, according to them, is essentially included in Islam.
Islam is sometimes divided under two heads of "Faith", or "Iman", and "Practical Religion", or "Din". Faith (Iman) includes a belief in one God, omnipotent, omniscient, all-merciful, the author of all good, and in Mohammed as His prophet, expressed in the formula: "There is no God but God, and Mohammed is the Prophet of God." It includes also, belief in the authority and sufficiency of the Koran, in angels, genii, and the devil, in the immortality of the soul, the resurrection, the day of judgment, and in the God's absolute decree for good and evil. Practical religion (Din), on the other hand, consists of five observances, viz.: recital of the formula of belief, prayer with ablution, fasting, almsgiving, and the pilgrimage to Mecca. For further details see KORAN and MOHAMMEDANISM.
The Catholic Encyclopedia, Volume VIII
Nihil Obstat, October 1, 1910. Remy Lafort, S.T.D., Censor
Imprimatur. +John Cardinal Farley, Archbishop of New York