Bhagavad Gita Verse 23, Chapter 9

yepyanyadevataa bhaktaa yajante shraddhyaanvitaahaa |
tepi maameva kaunteya yajantyavidhipoorvakam || 23 ||
 
Even those devotees who worship other deities, filled with faith, they also worship me only, O Kaunteya, (but) incorrectly.
 
ye : those
api : even
anyadevataahaa : other deities
bhaktaahaa : devotees
yajante : worship
shraddhyaa : faith
anvitaahaa : filled with
te : they
api : also
maam : me
eva : only
kaunteya : O Kaunterya
yajanti : worship
avidhipoorvakam : incorrectly
 
The recurring theme of this chapter is the removing misconceptions about the worship of Ishvara. Shri Krishna again invokes that theme in this shloka. He says that those devotees who worship deities other than the infinite Ishvara ultimately worship him, but do so in a wrong manner.
 
Many of us have been brought up in a tradition in which we worship a specific deity. As children, we are taught to invoke that deity during auspicious occasions, during periods of prosperity as well as periods of difficulty. We should be grateful to our parents for inculcating these good samskaaraas or habits in us at an early age. However, Shri Krishna says that as we grow older, it is important to have the correct knowledge of what we are worshipping, because in most cases, our knowledge is limited and incorrect.
 
What is this incorrect knowledge? Thinking that what we are worshipping is a finite deity in a certain form is incorrect knowledge. When we see a small government office, we do not make the mistake of thinking that a small office contains the government of an entire nation. Or when we look at a wave, we never imagine that the entire ocean is just that small wave.
 
Similarly, even though we worship a finite deity in our home or in a temple, we should never think that we are worshipping just that finite deity. If we think in that way, our worship will be incorrect, it will have a flaw.
 
So then, what is the right knowledge? It is knowing that we are worshipping Ishvara in his infinite nature. Just like we contact the entire Internet when we surf the web on our phone, we contact the infinite Ishvara when we worship a finite deity. Ishvara is the foundation of everything, therefore ultimately all prayers reach Ishvara.
 
Why do most devotees commit this error? This is explained next.