Divine Decree means that all things are known to Allah (Exalted be He) before their existence and that He writes them (in Al-Lawh Al-Mahfuzh - the Preserved Tablet). He, then, gives existence to what He wills and creates what He wills. There are four ranks of Divine Decree, all of which a person should believe in. A person is not a true believer in Divine Decree unless they believe in its four ranks. It is authentically reported that the Prophet (peace be upon him) answered Jibril (Gabriel) when the latter asked him about Iman (belief) by saying:
To believe in Allah, His Angels, His Books, His Messengers, the Day of Resurrection, and to believe in the Devine Decree whether good or bad.
Muslim in his Sahih.
It is authentically reported on the authority of
`Ubadah ibn Al-Samit that the Prophet (peace be upon him) said:
You will not find the taste of Iman until you believe in Devine Decree; and know that what has passed you by was not going to befall you; and that what has befallen you was not going to pass you by.
Ibn Taymiyyah (may Allah be merciful to him) explained the meaning of the Hadith mentioned above in Al-`Aqidah Al-Wasitiyyah, which we recommend you to refer to and study.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.