Islam is not a monolithic religion and there has not been a single universally accepted understanding of the religion since its creation. Sunni Islam is the largest sect comprising somewhere around 85%-90% of all Muslims in the world. Shia Islam makes up 10%-15% of the Islamic population of the world. The vast majority of Muslims are Sunni. “Sunni” comes from “Sunnah” which means “path”, and they are considered more orthodox (if that term can be applied here) than the other sects. I've heard many people say that Sunnis are nicer or more peaceful than the Shias and vice-versa. This is not an accurate representation of reality. There are peaceful Sunnis and peaceful Shias as well as violent Sunnis and violent Shias. Abu Bakr al-Baghdadi, the new caliph of the Islamic State is Sunni whereas the Iranians also calling for Israel and the West's destruction is 90%-95% Shia. It is simply not accurate to say one sect is violent while the other one is not. The Quran teaches that Mohammed's example is to be imitated, therefore they cannot escape violence lest they ignore his example.
Also to answer an objection before it is raised, violent jihad is a fixation for the minority of Muslims, most Muslims are peaceful and warm people. The issue is that there are 1.6 billion Muslims in the world and even if only 10%-20% (a fair estimate) are seeking to follow Mohammed's example we are still left with 160,000,000-320,000,000 who adhere to a violent ideology. If 20% is an accurate number then there are as many violent Muslims in the world as there are people living in the United States.
The division between Sunni and Shia began after Mohammed’s death when choosing his successor. The Sunnis wanted the most capable leader and the Shias wanted one of Mohammed's descendants to succeed him. Shias reject the first three Caliphs and acknowledge the fourth (Ali), whereas Sunnis acknowledge all of the first four (Abu Bakr [632-634], Umar ibn al-Khattab [634-644], Uthman ibn Affan [644-656], and Ali Ibn Abi Talib [656-661]) to be Mohammed’s legitimate successors. These are referred to as the Rashidun Caliphs or “the rightly guided Caliphs."
I am going to focus most of my time on the Sunni sect of Islam simply because it represents the majority of Muslims. To reiterate a point made above, there is not one sect that is violent and another that is not, they all read the same Quran which commands Muslims to follow Mohammed's example. The probability of there being violent Muslims (or who support violent Jihad) is equal to the probability of there being Muslims who are obedient to the Quran and Islamic texts.
These are the pillars which are perhaps the closest thing to Islamic unity to be found. Regardless of which sect of the religion one adheres to in order to be a Muslim these pillars must be acknowledged. The pillars are summarized in the Hadith of Gabriel which is included in Sahih Bukhari and Sahih Muslim. The 5 pillars below are the Sunni version, but these principles are found in both sects (Shias have 7 pillars on top of the Shahadah).
1) Shahadah: declaring there is no god except God, and Muhammad is God's Messenger
2) Salat: establishing daily prayers
3) Zakat: giving to the poor and needy
4) Sawm: fasting and self-control during the blessed month of Ramadan
5) Hajj: pilgrimage to Mecca at least once in a lifetime if he/she is able to
The Six Articles of Faith
These are the articles of faith, these are essential to Muslims along with the pillars, the pillars could be seen as the action and the articles of faith could be seen as the underlying belief. Both are necessary to be a Muslim.
1) The oneness of God, Tawhid
3) Revealed books (Torah, Psalms, and Gospel)
4) The Prophets
5) The Last Day or The Day of Judgement
6) God's decree
The pillars, the articles of faith and the Quran are agreed upon by both sects. There are variations between what Sunnis and Shias believe concerning the compilation of the texts, but they presently read the same Quran regardless.
The Quran was received by Mohammed over the course of 23 years beginning 609 and ending in 632 when he died. The Quran has 114 surahs (chapters) which each contain ayats (verses). The Quran is not chronological. It is arranged by the length of the surahs. Surah 1 is an introduction of sorts and the most widely quoted of the whole Quran, then surah 2 is the longest surah, surah 3 the second longest etc. Knowing the order of the surahs is important for understanding the book (http://www.missionislam.com/quran/revealationorder.htm). The Surahs are separated into Meccan and Medinan periods. The Meccan revelations were received in Mecca and the Medinan in Medina. One important thing to note about the order of revelation is that in the case of a contradiction there is an interpretive bent towards the later Medinan revelations giving them the more authority than those of Mecca. These are also the surahs that contain the most violence.
As I said above, the Sunnis believe in the 5 pillars of Islam and the Quran, and they share this with all Muslims. Where the Shias and others differ with the Sunnis is with what hadiths they accept as authoritative. There are so many verses in the Quran saying to "Obey Allah and the Messenger” (3:32, 132; 4:13, 59, 69, 80; 5:92; 8:1… etc), but Mohammed is not giving interpretation or insight in the Quran. The Quran, in Islamic thought, does not contain any of Mohammed’s words, but is supposedly Allah speaking verbatim. This leaves us with a question: if we’ve been reading the Quran and nothing else, what then does Mohammed command? The answer is that the sayings, teachings, and example (the commands) of Mohammed are found in the hadith writings. Hadith literature is considered to be inspired 2nd only to the Quran in Islamic theology. This means that the traditions and sayings of Mohammed are 2nd in value to the Muslim next to the Quran.
There are 6 collections in the Sunni Canon that are nearly universally accepted within their sect (Again, this is 85%-90% of all Muslims), they are listed below:
1) Sahih Bukhari, collected by Imam Bukhari (d. 256 A.H., 870 C.E.), includes 7275 ahadith
2) Sahih Muslim, collected by Muslim b. al-Hajjaj (d. 261 A.H., 875 C.E.), includes 9200 ahadith
3) Sunan Abu Dawood, collected by Abu Dawood (d. 275 A.H., 888 C.E.)
4) Jami al-Tirmidhi, collected by al-Tirmidhi (d. 279 A.H, 892 C.E)
5) Sunan al-Sughra, collected by al-Nasa'i (d. 303 A.H., 915 C.E.)
6) Sunan ibn Majah, collected by Ibn Majah (d. 273 A.H., 887 C.E.)
I’ve only been investing time in Sahih Bukhari as of now. It is ten volumes and very repetitive. Perhaps at some point I will begin to dig into the others. Here is an example from Sahih Bukhari:
Volume 4, Book 52, Number 46 :
Narrated by Abu Huraira
I heard Allah's Apostle saying, "The example of a Mujahid in Allah's Cause-- and Allah knows better who really strives in His Cause----is like a person who fasts and prays continuously. Allah guarantees that He will admit the Mujahid in His Cause into Paradise if he is killed, otherwise He will return him to his home safely with rewards and war booty."
From the Quran and the hadith there are exegetes known as "mufassir" who have written "tafsir." They are commentaries and explanations of the Quran using the hadith. These are comparable to Christian commentaries in that there are some that are more widely accepted than others. Most Christians would agree with Matthew Henry’s Concise Commentary of the Whole Bible. It is safe to say that Matthew Henry’s commentary represents the beliefs of a majority of Christians except for a few. There may be peculiar personal beliefs that Matthew Henry has in his commentary, but the whole is ultimately reliable. The same could be said of the tafsirs in Islam. They are generally accepted as representative of the Sunni religion as a whole, though there are exceptions. Some of the most widely accepted tafsir are listed below:
1) Tanwir al-Miqbas Known as (Tafsir Ibn Abbas); written by Abd-Allah ibn Abbas (d. 68/687).
2) Tafsir al-Tabari; written by Muhammad ibn Jarir al-Tabari
3) Ma'alim al-Tanzil by al-Baghawi
4) Al-Muḥarrar al-Wajiz by ibn Atiyyah
5) Tafsir ibn Kathir by ibn Kathir
6) Zad al-Masir by Abu'l-Faraj ibn al-Jawzi
7) Tafsir al-Karimir Rahman fii Tafsir Kalam al-Mannan by Abd ar-Rahman ibn Nasir as-Sa'di
Again, I’ve only dug into Tafsir ibn Kathir by ibn Kathir and not all of the rest, however Tafsir ibn Kathir is considered to be the most reliable of these seven. Here is an example from Tafsir ibn Kathir:
Surah 9:5, Quran (Yussuf Ali):
But when the forbidden months are past, then fight and slay the Pagans wherever ye find them, an seize them, beleaguer them, and lie in wait for them in every stratagem (of war); but if they repent, and establish regular prayers and practise regular charity, then open the way for them: for Allah is Oft-forgiving, Most Merciful. - Surah 9:5 - AT TAUBA
…[D]o not wait until you find them [idolators]. Rather, seek and besiege them in their areas and forts, gather intelligence about them in the various roads and fairways so that what is made wide looks ever smaller to them. This way, they will have no choice, but to die or embrace Islam[.] Vol. 4, P. 376; Sura 9:5–At-Tawbah
You may be asking why this is important. It is important to understand what Islam says about Islam so that we can avoid painting an unfair picture of any ideology or point of view. Knowing what Islam says about itself is the surest way to judge accurately what the religion believes and to measure it against objective Biblical truth. It is also important for me personally that others understand what texts I am quoting from and to know that the violent views are not an obscure minority within Islam, but the mainstream views as well. When I say that Islam is a violent and deceptive ideology I’ve not said so only from a Biblical perspective, but also from its own mainstream statements about itself. Islam, like every other false ideology, ultimately commits suicide when held up to objective truth and reason. Islam is clearly condemned by the Bible, but even without the Bible it is condemned by common sense and reason. The inherent ugliness of the religion is not an outsider's perspective, but clearly seen in the Quran, hadith, and tafsirs. This is ethically important because I don’t appreciate when people take what I believe, misunderstand it, then use it against me. Because of this I am doing the best I can to understand Islam from Islam’s perspective and the reality is even worse than I had initially thought. This doesn’t mean that I will only quote from Islamic texts--far from it, but these will likely be the texts that I will be using when I do. Feel free to search these resources independently to see if I have overstated their value and usage within Sunni Islam.
My hope is for Muslims to be saved from deception unto the knowledge of Jesus as the Son of God. This requires that they deny the Shahadah, reject Mohammed as a false prophet, and turn to Jesus, the one who died in their place and rose from the dead. He lives today and he is able to forgive their sins once and for all.