There are two causes for disbelief; either because of heedlessness or because of imitation of the forefathers. Allah (swt) says:

إِنَّا وَجَدْنَا آبَاءَنَا عَلَىٰ أُمَّةٍ وَإِنَّا عَلَىٰ آثَارِهِم مُّقْتَدُونَ

(“Indeed, we found our fathers upon a religion, and we are, in their footsteps, following.”) (Az-Zukhruf: 23)

It is blind imitation; they found their fathers worshiping idols and so they imitated them. But idol worship has not existed from the beginning because Adam (PBUH) conveyed to his children the way of righteousness, and his descendants initially believed in Allah (swt). Afterwards, heedlessness occurred, and people started worshiping idols and stones, so their children indulged in a blind tradition of following their fathers.

Hence, the verse says:

( أَوْ تَقُولُوا إِنَّمَا أَشْرَكَ آبَاؤُنَا مِن قَبْلُ وَكُنَّا ذُرِّيَّةً مِّن بَعْدِهِمْ ۖ أَفَتُهْلِكُنَا بِمَا فَعَلَ الْمُبْطِلُونَ ) (Or [lest] you say, “It was only that our fathers associated [others in worship] with Allah before, and we were but descendants after them. Then would You destroy us for what the falsifiers have done?“). It is not acceptable for a person to say: “My fathers were like this so I do that”. For that, Allah (swt) does not burden persons with religious obligations when they are  too young and still in the care of their parents, thus, they do not say: “This is how our fathers were.” The obligation is imposed on individuals as soon as they grow intellectually enough to make them capable of choosing between alternatives.