Answered by Ustadha Mahdiyah Siddique
Question:
If a Muslim does not believe in Taqdeer (Destiny) is he or she still a muslim?
Answer:
In response to your question;
One of the fundamental factors that Iman constitutes of is Destiny (Taqdeer), so for one to be a believer; belief upon this will also be mandatory.
In a Prophetic narration; Jabir 
narrates that the Prophet of Allah 
said: “You can never be a Believer; as long as you do not believe in Destiny — in its good and its bad — to such an extent that he believes whatever is to afflict him; he would not have escaped it and whatever is not to happen (misses him) will not occur (was not to ever afflict him)”. (Sunan Tirmidhi, Hadīth no. 2144, p.1867, Al-Kutub Al-Sitta: Darussalam.)
Only Allah Knows Best
Written by Ustadha Mahdiyah Siddique
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham