It's true that , "we always worship one god"
He himself the Supreme Paramatma manifests / assumes different forms himself . We can see this truth highlighted in our ancient Upanishads.
Here in the Shvetashvatara Upanishad - Adhyaya 6 - Shloka 11 - it's said that there is only one god , who resides in the hearts of all the creatures.
एको देव: सर्वभूतेषु गूढः सर्वव्यापी सर्वभूतान्तरात्मा।
कर्माध्यक्ष: सर्वभूताधिवस: । साक्षी चेता केवलो निर्गुणश्च
eko devaḥ sarvabhūteṣu gūḍhaḥ
karmādhyakṣaḥ sarvabhūtādhivāsaḥ sākṣī cetā kevalo nirguṇaś ca ।।
He is the one without a second, yet he is hidden in every being. He is
all pervasive, the Self of all. He gives to all beings the fruits of
their actions, and he is the support of all. He is the witness,
bestower of consciousness, without attributes and unconditioned.
Similarly in the Mudgalopanishad Khanda 3 - Shloka 1 - We can clearly see the concept of One God .Here what it says-:
एको देवो बहुधा निविष्ट अजायमानो बहुधा विजायते ।।3.1।।
Eko Devo Bahuda Nivishta Ajaayamano Bahuda Vijayaye
In this creation there is only one god ,who is manifisted himself in
different forms . He himself being unborn , assumes different forms.
So no matter which god we worship, we worship only one Paramatma ,who is without second. i.e.non dual.