The Noble Qur'aan is a record of the exact words revealed by Allah, the Mighty and Majestic, through the Angel Jibril to the Prophet Muhammad (peace and blessings be upon him). It was memorized by Prophet Muhammad (peace and blessings be upon him) and then dictated to his Companions and written down by scribes, who cross-checked it during the Prophet's lifetime. Not one word of its 114 Soowar (chapters) have been changed over the centuries; therefore, the Qur'aan is in every detail the unique and miraculous text which was revealed to Prophet Muhammad (peace and blessings be upon him) over fourteen centuries ago.
The Qur'aan, the last revealed Word of Allah, is the primary source of every Muslim's faith and practice. It details all the subjects which concern us as human beings such as wisdom, doctrine, worship, and law, but its basic theme is the relationship between Allah and His creation. At the same time it provides guidelines for a just society, proper human conduct, and an equitable economic system.
The Pillars of Islam are its basic foundation upon which it is established; they are five, mentioned in a narration by 'Abdullah Ibn Umar (may Allah be pleased with him) in which the Prophet Muhammad (peace and blessings be upon him) said:
"Islam is raised on five pillars: To establish the Shahaadah (testimony that there is nothing worthy of worship except Allah and Muhammad is the servant and messenger of Allah), establishment of prayers, payment of Zakaat, the fast of Ramadaan and Pilgrimage (to Makkah)..."
Shahaadah (Testimony of Faith)
"Ashadu ann laa ilaha ill Allah wa ashadu anna Muhammadur Rasul'Allah"
It is the decisive belief expressed on the tongue as though one sees with his or her own eyes the certainty of his or her firm conviction. Some of the benefits attained by the declaration of the great Shahaadah include the liberation of the heart and soul from slavery to the created, and from following paths other than the Messengers' path.
It is the worship of Allah by performing the obligatory duties in their right and perfect manner and on their prescribed times. Some of the benefits include the openness of the heart, contentment and joy, peace, and restraint from committing sins and wrongdoings.
It is the devoted worship to Allah by offering the obligatory prescribed amount of charity deducted from the wealth upon which Zakaat is due. Some of the benefits are purifying the self from immoral characteristics such as miserliness and greed through fulfilling the needs of Islam and Muslims.
It is the devoted worship of Allah by refraining from everything known to nullify the fast during the days of Ramadhaan, as well as the prescribed times throughout the year as per the narrations of Prophet Muhammad (peace and blessings be upon him).
It is the worship of Allah by traveling to the sacred House (Al-Ka'bah) to execute the rites of Hajj. Some of the benefits include the training of the self to expend physical and financial sacrifices in obedience to Allah, as well as preparation and awareness for the Day of Judgment. This is the reason why Hajj is considered as one kind of Jihad (struggle) in the cause of Allah.
The Deen of Islam is a way of life comprised of Aqeedah (matters of Faith) and Shari'ah (Laws, Legislation). The foundation of Aqeedah Al-Islamiyyah is the belief in:
This foundation is verified by evidences from the Book of Allah, the Qur'aan, and from the Sunnah of Allah's Messenger (peace and blessings be upon him). Allah says in the Qur'aan in Surah Al-Baqarah verse 177,
"It is not Al-Birr (piety, righteousness) that you turn your faces towards the east and (or) west in prayers; but Al-Birr is the quality of the one who believes in Allah, the Last Day, the Angels, the Book, the Prophets."
In the Sunnah of Allah's Messenger, the Prophet Muhammad (peace and blessings be upon him) said in his reply to Jabriel when the Jibril asked him about eemaan (faith).
"Eemaan is to believe in Allah, His Angels, His Books, His Messengers, and the Last Day, and to believe in Al-Qadar, its good and its bad." - reported in Saheeh Muslim
Reference: Explaining the fundamentals of Faith by Sheikh Muhammad Bin Saleh Al-Uthaymeen