Salam Dear Mohammad,
Thank you for your question and for contacting Ask About Islam.
The Islamic belief system is based on six articles without which one’s belief is not complete. A Muslim is a person who believes in Allah, the Angles, the Scriptures, the Prophets, the Last Day, and the Divine Destiny. This is a complete package. If one item is missing, the faith process is incomplete.
Each article of faith has its own conditions. When you believe in Allah, you have to believe in His Divine Names and Attributes. You have to believe in His Mighty and Power. You have to believe that He controls everything on this universe.
You have to believe that He is over watching us day and night. Even is it not enough to believe in Allah as the Lord of the universe. You have to believe in Him as the only deity worthy of worship.
Belief in Allah as the lord of the universe was the belief of the people during the lifetime of the Prophet. Read for example, Al-Ankabut: 61; Al-Ankabut: 63; Al-Zukhruf: 87; Al-Muminun: 84-89; Yunus: 31; etc. Despite their belief in Allah as the Lord, they were considered unbelievers because of their disbelief in Allah as the one worthy of worship.
Coming to the belief in angels, one has to believe in their existence, their names which have been mentioned in the Quran, their characteristics, that they obey Allah and never disobey Him.
When you believe in the Prophets, you have to believe in Prophet Muhammad (peace and blessings be upon him) as the final and last messenger sent by Allah to guide humanity. You also have to believe in all the Prophets who were sent before him from Adam to Jesus (peace be upon them all). If you do not believe in any of them, your faith is incomplete.
You have to believe that they are all on equal footing and that Allah does not differentiate between any of them (Al-Baqarah: 285) and that we should not believe in some of them and deny some (Al-Baqarah: 85).
With regard to belief in the Scriptures, one has to believe in all the Books that were revealed to the Prophets before Prophet Muhammad and were mentioned in the Quran such as the original copies of the Torah, the Injeel, the Scrolls of Abraham, the Psalms of David.
As for belief in the Last Day, one has to believe that this life is temporary and one day it will come to an end. We have to believe that all people will be resurrected and will stand before Allah and be held accountable for what they did in this life.
The same holds true for belief in the divine destiny. One has to believe that everything is determined by Allah and that we act upon this determination with our free will.
In addition to the six articles of faith, there are other five pillars of Islam, namely; to testify that there is no deity worthy of worship except Allah, that Muhammad is the messenger of Allah, to perform the five daily prayers, to pay the obligatory charity, and perform hajj. Each pillar also has it conditions and details.
The Prophet is reported to have said:
“He who testifies that there is no god but Allah and that Muhammad is the messenger of Allah, Allah would prohibit the fire of Hell for him.” (Muslim)
“If anyone testifies (sincerely from his heart) that there is no god but Allah, and that Muhammad is His bondsman and His messenger, Allah immunes him from Hell.” (Muslim)
“He who testifies that there is no god but Allah and I am the messenger of Allah would not enter Hell or its (flames) would not consume him.” (Muslim)
I hope this answers your question. Please keep in touch.