Ibrahim, peace be upon him, knew that they were idol worshippers. However, he asked them to show them that what they worship is not deserving of worship in any way. It is like saying to a merchant: What is your wealth? While you know that his wealth is slaves. Then you say to him: Slaves are not wealth. If you say: What do you worship? It is merely a question about the object of worship. The expectation would be for them to say: Idols, as Allah, the Most High, says: 'And they ask you: What should they spend? Say: The excess.' What did your Lord say? They said: The truth. What has your Lord revealed? They said: Goodness. I say: These people have come with their story complete, as if they are rejoicing in it and boasting about it. It included Ibrahim's response and what they intended to reveal from their feelings of joy and pride. Do you not see how they added to their saying: 'We worship' and then said: 'So we will remain devoted to them,' and did not limit themselves to just saying: 'We worship'? An example of this is when you ask some clever people: What do you wear in your country? They might say: I wear the thick cloak, and I drag its hem among the maidens of the neighborhood. They said: 'We will remain,' because they used to worship it during the day and not at night.