Tafsir of Al-Mu'minoon 23:90

بل أتيناهم بالحق

Nay, but We have brought them the truth, which is the declaration that there is no god worthy of worship besides Allah, and the establishment of clear, definitive and sound proof to that effect,

وإنهم لكاذبون

and verily, they are liars. means, in their worship of others alongside Allah when they have no evidence for doing so, as Allah says at the end of this Surah:

ومن يدع مع الله إلها آخر ﻻ برهان له به فٳنما حسابه عند ربه

إنه ﻻ يفلح الكافرون

And whoever invokes, besides Allah, any other god, of whom he has no proof; then his reckoning is only with his Lord. Surely, the disbelievers will not be successful. (23:117) The idolators have no evidence for what they are doing, which has led them into lies and misguidance. Rather they are following their forefathers and predecessors who were confused and ignorant, as Allah describes them:

إنا وجدنا ءاباءنا على أمة وإنا على ءاثرهم مقتدون

"We found our fathers following a certain way and religion, and we will indeed follow their footsteps.'' (43:23)