No father is accountable for his child’s wrongdoing, and no child for his father’s wrongdoing
Question

                I have a question abou this verse:
Al kahf - ayah 5
They (Christians) have no knowledge of it, nor had their fathers. Grave is the word that comes out of their mouths; they speak not except a lie.
What do they mean with nor had their fathers. For example if the son of a sheikh who knows every single word of the quran converted to Christianity. Does that mean his father has no knowledge?
Praise be to Allah.Allah, may He be glorified and exalted, says (interpretation of the meaning):
“[All] praise is [due] to Allah, who has sent down upon His Servant the Book and has not made therein any deviance.
[He has made it] straight, to warn of severe punishment from Him and to give good tidings to the believers who do righteous deeds that they will have a good reward
In which they will remain forever
And to warn those who say, " Allah has taken a son." ’
They have no knowledge of it, nor had their fathers. Grave is the word that comes out of their mouths; they speak not except a lie. ”
[al-Kahf 18:1-5].
The words “And to warn those (Jews, Christians, and pagans) who say, ‘Allah has begotten a son (or offspring or children)” refer to the Jews, Christians and polytheists who uttered these abhorrent words. They were not speaking on the basis of knowledge or certainty, whether that was their own knowledge or knowledge from their forefathers whom they were imitating and following. Rather they were following nothing but conjecture and their own whims and desires.
“Mighty is the word that comes out of their mouths” that is, it is mightily abhorrent and the punishment for it will be severe. What can be more abhorrent than describing Him as having begotten offspring, which would imply imperfection on His part, association of another with Him in the unique attributes of Lordship and divinity, and telling lies against Him?
Hence Allah says: “They utter nothing but a lie” that is, it is pure lies in which there is no truth at all.
Think about how the Quran nullifies this view gradually, moving from one thing to another that is even more false. First of all Allah says, “No knowledge have they of such a thing, nor had their fathers”. Undoubtedly speaking about Allah without knowledge is prohibited and false. Secondly, He states that this is a mightily abhorrent thing to say, as He says: “Mighty is the word that comes out of their mouths.”
Then, thirdly, He describes the degree of offensiveness, which is a lie that is contrary to truth.
See: Tafseer as-Sa‘di (p. 469).
What is meant by the words “No knowledge have they of such a thing, nor had their fathers” is that they spoke words of falsehood concerning which they had no knowledge, and neither did their fathers who preceded them in uttering these false words. Then their children came and followed their fathers in that. This is something that neither they nor their fathers from whom they learned it knew or understood.
What is meant by fathers here is their forefathers who spoke these false and corrupt words. They were speaking on the basis of speculation and their own whims and desires, and their children followed them in that. So it is false words that were uttered by the sons without knowledge, following their ignorant, lying forefathers in that. How terrible it is for a man to be ignorant and to follow others in ignorance.
Ash-Shawkani (may Allah have mercy on him) said:
They had no knowledge of that at all, and neither did their fathers have knowledge. Rather, according to their own claim, they were following misguidance and their children followed them, so all of them went astray. End quote.
Fath al-Qadeer (3/320)
The scholars have stated that what is meant by fathers here is the fathers who spoke these corrupt words, and their children learned them from them.
Ibn ‘Atiyyah (may Allah have mercy on him) said:
The words “nor had their fathers” mean: those from whom these people took this view. End quote.
Tafseer Ibn ‘Atiyyah (3/495).
Ash-Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
“nor had their fathers” who said such words. They had no knowledge concerning that; it was nothing but illusions that they thought were real, and that is not knowledge. End quote.
Tafseer al-‘Uthaymeen/al-Kahf (p. 13)
Ibn ‘Ashoor (may Allah have mercy on him) said:
They used to say “We found our fathers following a certain way and religion, and we will indeed follow their footsteps” [az-Zukhruf 43:23]. If their fathers did not have any proof for what they said, then they did not deserve to be followed. End quote.
At-Tahreer wa at-Tanweer (15/251)
As for fathers who do not believe in such corrupt views, rather they believe in Allah and affirm His Oneness (Tawheed), they are not referred to in this verse.
If someone is a believer in the Oneness of Allah, and he has a son who apostatises from Islam and becomes a Christian, and begins to say that the Messiah is the son of God, that monotheist father is not to be criticised or blamed, and he is not to be described as lacking in knowledge because of what his son did. Rather every individual is to be criticised or praised on the basis of what he himself does, not what anyone else does.
Abu Dawood (4495) narrated that Abu Rimthah said: I went to the Prophet (blessings and peace of Allah be upon him) with my father, then the Prophet (blessings and peace of Allah be upon him) said to my father: “Is this your son?” He said: Yes, by the Lord of the Ka‘bah. He said: “Is it true?” He said: I bear witness to it. The Messenger of Allah (blessings and peace of Allah be upon him) smiled at my resemblance to my father and my father’s oath concerning me, then he said: “You are not accountable for his wrongdoing and he is not accountable for yours.” And the Messenger of Allah (blessings and peace of Allah be upon him) recited the verse: “no bearer of burdens shall bear the burden of another” [al-An‘am 6:164]
Classed as saheeh by al-Albani in Saheeh Abi Dawood.
Al-Qari (may Allah have mercy on him) said in Mirqat al-Mafateeh (6/2272):
“You are not accountable for his wrongdoing” i.e., you are not responsible for his sin; “and he is not accountable for yours” i.e., he is not responsible for your sin. End quote.
Al-‘Ayni (may Allah have mercy on him) said:
Thus the Messenger of Allah (blessings and peace of Allah be upon him) confirmed what Allah stated, that each person is responsible for his own wrongdoing, just as his good deeds are to his own credit, not the credit of anyone else. End quote.
‘Umdat al-Qari (8/79).
And Allah knows best.