Q 21: What is the textual proof from the Glorious Qur'an and the Prophetic Sunnah (Tradition) that a Muslim must have absolute faith?
A--- Allah the Almighty says: {Only those are the believers who have believed in Allah and His Messenger, and afterward doubt not but strive with their wealth and their lives for the Cause of Allah.
Those! They are the truthful}. (Al-Hujurat, 15)
Prophet Muhammad (Peace be upon him) says: "I bear testimony to the fact that there is no god but Allah, and I am His messenger. The slave who would meet Allah without entertaining any doubt about these (two fundamentals) would enter heaven ". (Muslim)
Prophet Muhammad (Peace be upon him) said to Abu Hurairah (May Allah be pleased with him): "Take these sandals of mine, and when you meet anyone outside this garden who testifies that there is no god but Allah, being assured of it in his heart, gladden him by announcing that he shall go to Paradise ". (Sahih Muslim)
Reference: 200 FAQ On Muslims Belief - Shiekh Hafiz Ibn Ahmed Al Hakami