In the Bh.Gita 9th chapter Bhagavan says:
त्रैविद्या मां सोमपाः पूतपापा यज्ञैरिष्ट्वा स्वर्गतिं प्रार्थयन्ते ।
ते पुण्यमासाद्य सुरेन्द्रलोकमश्नन्ति दिव्यान्दिवि देवभोगान् ॥ २० ॥
9.20 Those who are versed in the Vedas, who are drinkers of Soma and are purified of sin, pray for the heavenly goal by worshipping Me through sacrifices. Having reached the place (world) of the king of gods, which is the result of righteousness, they enjoy in heaven th divine pleasure of gods.
Shankara introduces that verse thus:
ये पुनः अज्ञाः कामकामाः — मां वस्वादिदेवरूपिणं
Those, again, who are ignorant and desirous of pleasures. Shankara says that these people worship Bhagavan, existing in the forms of gods such as the Vasus and others; yajnaih, through sacrifices such as the Agnistoma etc.
Subsequently Bhagavan says:
येऽप्यन्यदेवता भक्ता यजन्ते श्रद्धयाऽन्विताः।
तेऽपि मामेव कौन्तेय यजन्त्यविधिपूर्वकम्।।9.23।।
9.23 Even those who, being devoted to other deities and endowed with faith, worship (them), they also, O son of Kunti, worship Me alone (though) following the wrong method.
Shankara introduces this verse thus:
ननु अन्या अपि देवताः त्वमेव चेत् , तद्भक्ताश्च त्वामेव यजन्ते । सत्यमेवम् —
'If you Yourself are the other gods as well, then do not their devotees too worship You alone?' 'Quite so!'
Here, drawing from Bhagavan's own words in 9.20 that those desirous of worldly pleasures propitiate Me alone through sacrifices, yajna-s, Shankara asks as above: you Yourself are the other gods as well.
Thus, from the words of Bhagavan Himself we understand that those who are worshiping other deities are actually worshiping Him, Brahman. Those deities are not different from Him, Brahman.
'anya devata' = those deities 'other' than one's own Self, Atman. We see from the puranas and the Mahabharata that those worshiping even Vishnu through sacrifices go to Vishnu loka and return. Bhagavan says in the very same chapter of the Gita:
यान्ति देवव्रता देवान् पितृ़न्यान्ति पितृव्रताः।
भूतानि यान्ति भूतेज्या यान्ति मद्याजिनोऽपि माम्।।9.25।।
9.25 Votaries of the gods reach the gods; the votaries of the manes (pitrus) go to the manes; the worshippers of the Beings reach the Beings; and those who worship Me reach Me. Here Bhagavan implies that 'votaries of gods reach gods' the deity Vishnu is also one of those gods.
Since those who worship 'other' deities do not reach Bhagavan and only those who consciously seek Him attain Him, and also from the statements of other Purana and Mahabharata that even Vishnu is worshiped with worldly ends as the goal, we conclude that the statement made in the Gita is by Brahman and not the deity.
Also we have in the Aitareya Upanishad: एष ब्रह्मैष इन्द्र एष प्रजापतिरेते सर्वे देवा 3.5 He is Brahma, He is Indra, He is Prajapati, He is all these gods...
This pramana is accepted and cited by Shankara while determining the status of Hiranyagarbha.
From all this we arrive at the conclusion that: All deities are actually Brahman alone.
Om Tat Sat
subbu