Steadiness of the mind is obtained by image worship. The worshipper will have to associate the idea of infinity, omnipotence, purity, perfection, freedom, holiness, truth and omnipresence. It is not possible for all to fix the mind Absolute. A concrete form is necessary for practicing concentration. To behold God everywhere and to practice the presence of God is not possible for the ordinary man.
Idol worship is the easiest form of worship for the modern man.
Idol worship is not just limited to Hinduism. Christians worship the Cross. They have the image of Cross in their mind. The Muslims keep the image of the Kaaba stone when they kneel and do prayers. The people of the whole world keep some image or the other in their mind.