The Lord's Deity forms, manifested in material elements, are actually fully spiritual incarnations. If one sincerely and faithfully worships the Deity under the guidance of a bona fide spiritual master, the Deity will eventually reveal Himself as the Supreme Personality of Godhead. But for those who have no faith, the Deity will always appear to be a material object.
Being unlimited, to bless His devotees the Lord can appear even in material elements. Nonetheless, it is not that simply any material form may be worshiped-only forms described in the scriptures. For example, to post a letter we must place it into a mailbox, not just any box lying on the street. Because the postal department has authorized the mailbox, our letter is sure to reach its destination.
Similarly, when one offers prayers to the Lord and worships those forms of His that are recommended in Vedic scriptures, the Lord reciprocates and one advances in spiritual realization.
