The reality of sin is not that Allah first created it and then thousands of years later conjured up the idea of forgiveness. For example, a fly has two wings, one containing poison and the other its antidote. Similarly, human beings have two wings, one of sin and the other of shame, repentance and remorse. This is a general rule. For example, if someone were to violently beat a slave, remorse would overwhelm such a one. It is as if both wings move in unison. Poison is not without its antidote. Now the question is, why was this poison created in the first place? The answer is that although this is a poison, if harnessed, it possesses remedial properties. If sin did not exist, the venom of vanity would surge forth within man and lead to his ruin, but repentance prevents this from happening. Sin protects man from the calamity of arrogance and conceit. When the Prophet Muhammadsa would seek forgiveness seventy times in one day – despite being sinless – how great an effort should we exert? Only one who is content with sin does not repent; an individual who considers sin to be evil will ultimately refrain from it.
It is narrated in a Hadith that when a person weeps before Allah time and again to seek forgiveness, ultimately, God says, “We have forgiven you, now do as you please.” This actually means that the heart of such a one has been transformed and thereafter he shall feel an innate aversion towards sin. No one who observes a sheep eating filth is driven by greed to consume it as well. Similarly, a person who is forgiven by Allah will never commit sin. Muslims instinctively abhor the flesh of swine though they may be indulged in thousands of other unlawful and forbidden things. The wisdom in placing this form of disgust in Muslims is to illustrate that mankind should develop a similar revulsion towards sin.
(Hazrat Mirza Ghulam Ahmadas, Malfuzat, Vol. 1, pp. 3-4)