All praise is due to Allah, and peace and blessings be upon our Master, the Messenger of Allah.
Dua` is a cause among causes, just as striving to earn a living is a cause for sustenance, and just as marriage is a cause for children, and indeed, just as steadfastness in obedience is a cause for entering Paradise.
The fact that these things are causes means that Allah, the Exalted, has made them a path to reach the cause. Whoever does not follow this path will not achieve their goal. This does not contradict a Muslim's belief in Allah's decree and predestination.
This is like the example of a teacher who demands that his students be diligent and hardworking and pass the exam to obtain a certificate. The teacher knows who will succeed and who will fail among his students, but the reward cannot be based on the teacher's knowledge, rather it must be based on the cause (i.e., the student's passing or failing the test).
Imam Al-Ghazali (may Allah have mercy on him) said: "If it is asked, 'What is the benefit of Dua` when divine decree cannot be averted?' One must know that among the decrees of Allah is the repelling of affliction through supplication. Supplication is a means to repel affliction and bring about mercy, just as a shield is a means to repel weapons, and water is a means for vegetation to grow from the earth. Just as a shield repels an arrow, and they counteract one another, so too do Dua` and affliction counteract each other. Allah has decreed the matter and decreed its cause. This carries benefits, as we have mentioned: the presence of the heart and a state of neediness before Allah, which are the pinnacle of worship and knowledge. And Allah The Almighty knows best. "
Ibn al-Qayyim (may Allah have mercy on him) said: “What is decreed is decreed alongside its causes, and among its causes is supplication. It is not decreed in isolation from its cause; rather, its cause is also decreed. When a person brings forth the cause, what is decreed comes to pass, and if the cause is absent, what is decreed does not occur. For example, Allah has decreed that satiety and quenching come through eating and drinking, that offspring come through sexual intercourse, and similarly, that entering Paradise comes through righteous deeds and entering Hellfire through sinful deeds. Thus, supplication is one of the most powerful causes. If the occurrence of what is being supplicated for has been decreed to come about through supplication, it is incorrect to say that supplication has no benefit—just as one cannot say there is no benefit in eating, drinking, or any other actions or movements. There is nothing more beneficial among the causes than supplication, nor anything more effective in attaining what is sought.” And Allah The Almighty knows best.
