Iman (Faith) means to believe in the heart, to confess by the tongue and to act with the parts of the body.
By some words and deeds it may increase and by some words and deeds it may decrease.
The pillars of Iman are six (i.e. to believe in):
1. Allah (SWT).
2. His Angels.
3. His Messengers.
4. His Books.
5. The Last Day.
6. Divine Preordainments, good or bad.
The Belief in Allah (SWT) is that you should believe that Allah (SWT) is the Sole Creator, Sustainer, Provider and the One in Whose Hand is the disposal of all affairs. Everything stands in need of Him, but He stands in need of none. He is the Only One Who is worthy of being worshipped. He has the Best Names and Perfect Attributes.
The Angels are the creatures of light. They are Allah's obedient slaves, they do that which they are commanded and are incapable of disobedience.
It means that Allah (SWT) sent the Messengers like Musa (Moses), Isa (Jesus), Ibrahim (Abraham), Nuh (Noah), etc. (peace be upon them all) and sent down the books like the Torah, Injeel (Gospel), Zaboor (Psalms), etc. to call the people to worship Allah (SWT) Alone, associating nothing with Him. He sealed (finalized) the Messengers with Prophet Muhammad (SAW) and abrogated all previous books with the Quran. Therefore the worship should be done according to the Quran and the Sunnah of Prophet Muhammad (SAW).
What is meant by Belief in Preordainment (Qadar)?
The Belief in Preordainment (Qadar) means to believe that everything - good or bad - happens or takes place according to what Allah (SWT) has ordained for it. He has created everything in due proportion.