Mayavadis and atheists accept the forms of the Deities in the temple of the Lord as idols, but devotees do not worship idols. They directly worship the Personality of Godhead in His arca incarnation. Arca refers to the form we can worship in our present condition. Although Kṛṣṇa is beyond our vision, He has agreed to be seen by us through the arca-vigraha, the Deity. We should not think that the Deity is made of stone. Even if it is stone, we should think that Kṛṣṇa has made Himself visible before us like a stone because we cannot see beyond stone. That is Kṛṣṇa’s mercy. Because our eyes and other senses are imperfect, we cannot see Kṛṣṇa present everywhere in His original spiritual form. Because we are imperfect, we see the difference between things spiritual and material, but Kṛṣṇa, being absolute, knows no such distinctions. He can become spiritual or material, however He likes, and it does not make any difference to Him. Being almighty and omnipotent, Kṛṣṇa can change matter into spirit and spirit into matter. Therefore we should not think, as the atheists do, that we are worshiping idols. Even if it is an idol, it is still Kṛṣṇa. That is the absolute nature of Kṛṣṇa. Even if we think that the Deity is a stone, or a piece of metal or some wood, He is still Kṛṣṇa. The understanding of this requires bhakti on our part. If we are a little thoughtful and philosophical, and if we are at all inclined toward bhakti, we can understand that Kṛṣṇa is present in stone.
There are many sastric injunctions which give instructions for carving forms of the Lord. These forms are not material. If God is all-pervading, He is also in the material elements. There is no doubt about it. But the atheists think otherwise. Although they preach that everything is God, when they go to the temple and see the form of the Lord, they deny that He is God. According to their own theory, everything is God. Then why is the Deity not God?
Source : A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Teachings of Queen Kunti”, Page 90 & 91
A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “The Science of Self-Realization”, Page 21