What is a convincing response to someone who says that fasting on the Day of Arafah is not required every year, given that it expiates the sins of two years?

The answer to the posed question can be summarized in several points:

1. Completeness of Righteous Deeds: Atonement for sins through righteous deeds is conditional upon the complete performance of the deed. Any deficiency in it reduces the reward and atonement, as is the case with prayer and fasting. If the fasting was incomplete, the Muslim may need to fast in the following year to atone for the remaining sins. 2. Mitigation of Major Sins and Increase in Reward: Even if the fasting was complete and atoned for sins, fasting in the following year can alleviate major sins (if any) or elevate one's status and earn more good deeds. This is not exclusive to the Day of Arafah but includes many righteous deeds that atone for sins. 3. Multiple Virtues of Fasting: The virtues of fasting are not limited to atoning for sins only, but also include refining the soul, fostering piety, receiving abundant multiplied reward from Allah, and other virtues that encourage a Muslim to fast, even if it is assumed that their sins have been forgiven.