Everyone knows that this mūrti is made of stone or wood or metal. But it is not that. Kṛṣṇa is all-powerful. He can take your service in any way. But the point is that because you cannot see beyond this metal and wood and stone, therefore He comes in that form, so that you can see. This is the point.
Don't be carried away by the nonsense philosophy that "These people are worshiping idol." It is not idol. It is same example: He is Kṛṣṇa Himself. Always same. He is Kṛṣṇa Himself. He has come to you for . . . just you can see Him. That's all. Otherwise Kṛṣṇa, ataḥśrī-kṛṣṇa-nāmādi na bhaved grāhyam indriyaiḥ (CC Madhya 17.136)—with these blunt eyes, material eyes, you cannot see Kṛṣṇa at the present moment. So whatever you can see, Kṛṣṇa has conveniently has taken His form in that way. So you should always think that Kṛṣṇa is present here—not that wood or stone. Then you are . . .
(aside) Go on.
Directly, arcye śilā-dhīr guruṣu nara-matiḥ vaiṣṇave jāti-buddhiḥ (Padma Purāṇa), these are forbidden. Never think of the arcā-mūrti as wood or stone—never think of the spiritual master as ordinary human being—never think of a Vaiṣṇava as belonging to a certain caste or nation.