(وَإِذَا فَعَلُوا فَاحِشَةً) (And when they commit an immorality): The Arabic word used for (immorality) comes from the word (tafahosh), meaning: (to become viler). According to scholars, this word is used in the Holy Quran with a specific type of sins, and that is adultery. Allah (swt) says:
وَلَا تَقْرَبُوا الزِّنَا ۖ إِنَّهُ كَانَ فَاحِشَةً وَسَاءَ سَبِيلًا
(And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way.) (Al-Israa: 32)
It is used for adultery because unlike other sins where their effects do not transgress onto others than the one who committed the sin, but in the case of adultery it transgresses onto others. After adultery there will either be a killing of the child who is born out of adultery, or there will be a fatherless child, and mixing up of ancestries, together with others sins that follow adultery. This is why it is considered to be the vilest immorality.
( قَالُوا وَجَدْنَا عَلَيْهَا آبَاءَنَا وَاللَّهُ أَمَرَنَا بِهَا) (they say, “We found our fathers doing it, and Allah has ordered us to do it.”): Here we find two points, the first one is the blind imitation represented by saying (We found our fathers doing it,). The second is their claim that Allah’s (swt) commanded them of it. All Divine Religions were sent to abolish blind imitation. Allah (swt) did not answer their claims about imitation because it is an invalid argument, while He (swt) answered the second claim: (and Allah has ordered us to do it.”) by saying:
( قُلْ إِنَّ اللَّهَ لَا يَأْمُرُ بِالْفَحْشَاءِ ۖ ) (Say, “Indeed, Allah does not order immorality): the reason is found in the following verse:
إِنَّ اللَّهَ يَأْمُرُ بِالْعَدْلِ وَالْإِحْسَانِ وَإِيتَاءِ ذِي الْقُرْبَىٰ وَيَنْهَىٰ عَنِ الْفَحْشَاءِ وَالْمُنكَرِ وَالْبَغْيِ ۚ يَعِظُكُمْ لَعَلَّكُمْ تَذَكَّرُونَ
(Indeed, Allah orders justice and good conduct and giving to relatives and forbids immorality and bad conduct and oppression. He admonishes you that perhaps you will be reminded.) (An-Nahel: 90)
( أَتَقُولُونَ عَلَى اللَّهِ مَا لَا تَعْلَمُونَ) (Do you say about Allah that which you do not know?): They did three wrong things: Immortality, blind imitation, and talking about Allah (swt) with ignorance by claiming that Allah (swt) commanded them of these vile acts.