Question: It is said, "No thawab [reward] is given for any of the good deeds of a person not performing salat. Similarly, the acts of worship offered by a person committing haram are not accepted." Why does Allah let our acts of worship go to waste?
ANSWER
Allahu ta'ala does not let good deeds go to waste, nor does He wrong anyone. Not performing salat is one of the most serious sins. That is, it is haram. The meaning of the statement "The acts of worship of a person who commits haram are not accepted" is that such a person cannot earn the great rewards (thawabs) promised in return for acts of worship. In other words, it means that he cannot keep all his rewards because his sins will reduce his rewards. It does not mean he is not given any rewards. Every act of worship brings rewards, but the harams a person is committing wipe out his rewards. Let us suppose that 70 units of thawab are given to a fasting person and 70 units of sins are written for a person drinking alcohol. If one breaks one's fast with alcohol at sunset, one earns 70 units of thawab in return for fasting and is burdened with 70 units of sins as well. As a result, one is left without any thawabs. If one had not fasted, one would have remained with the sin of drinking alcohol. Does it not suffice that one's fast has caused one's sin of drinking alcohol to be forgiven? Therefore, a sinner should not be negligent in doing his acts of worship. If one has committed multiple sins, they will decrease one’s thawabs, and the result may be a minus quantity. Not performing a salat is, say, 1,000 thousand units of sins. No matter how many good deeds and acts of worship one may offer, one cannot neutralize this amount. As voluntary acts of worship, when compared with obligatory ones, are not even like a drop of water compared to sea, the sins that one has accumulated because of abandoning obligatory acts of worship efface these good deeds, thus rendering one without any good deeds. What I have tried to explain here is the meaning of the statements “No reward is given for any of the good deeds of a person not performing salat” and “The acts of worship offered by a person committing haram are not accepted.”

The same explanations apply to the statement “The voluntary salats of one are not accepted if one has missed salats.” When one delays something obligatory and busies oneself something voluntary instead of it, one’s voluntary salats will go to waste because the sin of delaying an obligatory act of worship is more than the thawab of offering a voluntary act of worship. One certainly earns rewards in return for offering a voluntary salat, but one’s loss outweighs one’s gain. The reason why this is so is that voluntary acts of worship, when compared with compulsory ones, are not even like a drop of water as compared to sea.

All the pieces of information mentioned above concern a Muslim who follows the aqidah of Ahl as-Sunnah wa’l Jama’ah. A person who does not follow this aqidah is called ahl al-bid’ah (a person of innovation). If such a person’s acts of worship are valid, he is considered to have carried out his duties and will not be punished for them. However, he will not be able to receive the rewards promised in return for these acts of worship and will not be requited in the hereafter for the good and charitable deeds he did in the world. (Miftah-ul Jannah)

The good deeds of disbelievers and people of innovation will be rejected, and they will be punished for their evil deeds.