How to steal the idol in Baldur's Gate 3 Druid Grove

The idol of Silvanus is located on the stone steps in the center of the grove and is worshiped by the druids who perform the ritual to close the grove. You can approach and interact with the idol, but then be prepared for the consequences. The druids will immediately notice and accuse you of stealing, and attack the tieflings as well. A massive battle will begin.

It's much better to do this later, after you've dealt with the goblin camp and hosted a party (this option might not be available anymore). Then the tieflings will leave the Grove, and the idol may end up inside the cave, from where it will be much easier to steal it.

You can steal from outside, but use magic to hide yourself. For example, a Shadow on a character who steals an idol, or a misty cloud around it. The druids will probably turn to you anyway, suspecting you of the theft. Pass a skill test. Still, it's better to do just that and do it before the party in honor of the victory over the goblins.