What makes the Christian trinity monotheistic but the Hindu trinity polytheistic?
Christians believe that God is The Father, The Son and the Holy Spirit, and that these three are just one God and they call that monotheism.
Hindus see all being as Brahman, and Brahman is also Brahma, Vishnu and Shiva, yet all three are ultimately Brahman, and Christians, if they even know about that, call it polytheism although it works the same way.
Do Christians call their polytheism "monotheism" to make it sound better? Honestly, I don't see any difference between Father/Son/HolySpirit=God and Brahma/Vishnu/Shiva=Brahman at least as far as the monotheistic vs polytheistic thing goes.
Hindu Scripture says;
"He is One only without a second."
[Chandogya Upanishad 6:2:1]
Christian Scripture says;
"The Lord our God is One Lord"
[Mark 12:29]
Hindus have their trinity (Trimurti) but also have thousands of other Gods too, whilst Christianity only has the Trinity. Also, the Trimurti is seen as three separate Gods, in Christianity all three together are one God, three beings, but not Gods. Also in Hinduism, after the Trimurti, there's Brahman (different to Brahma) as Brahma is one being/God, but Brahman is every single Hindu God in one to form a single entity
The only real difference is that, outside of Catholicism, the trinity is the end of the line. In Hinduism, there is a pantheon of other deities, whether or not in the end they are all aspects of Brahman. Catholicism is, in my opinion, a polytheistic religion as well, because they pray to God, Jesus, Mary, and countless other saints and they consider the Pope infallible. But mainstream Christianity only follows God, Jesus the Son, and the Holy Spirit as aspects of one being. I guess the claim to monotheism is tenuous, at best, but because they do not branch out any further they have a closer claim to monotheism than Hinduism, in their eyes.
Most peoples' perception of Hinduism is that there are thousands of gods. They do not realize that Hindus actually believe in ONE tribune God and that the other "gods" are different manifestations in which the One God has shown Himself throughout time. Hindu philosophy is extremely deep.
If some Hindus say that they worship one God, I'll believe them. I'm a Christian and believe that I worship one God. So I'd be a hypocrite if I get annoyed at Muslims saying that Christian worship three Gods, and then turn around and call Hindus liars.
You are just scratching the surface. Please read more and you will understand.
Hinduism says God is ONE without any attributes or qualities. It is beyond human cognition of the senses and mind and can only be realized by meditation and self realization which is called Yoga.
It also says that God can appear in many forms which are all manifestations of the formless God. The manifested form is a deity which is worshiped.
Similar concepts in other areas:
Energy is ONE concept but has many forms.
Money is ONE concept but has many forms.
A person is ONE but has many roles in life.
Life is ONE but has 3 phases - Birth, Growth and Death. This is the concept of trinity.
Similar trinity applies to all ideas and so many other things in human experience.
The true: God Father, Lord Jesus, and Holy Spirit are one and same Deity/Being, but Brahma, Vishnu and Shiva are just some false gods/idols/unclean spirits, all Hindu faiths/religions are satanic delusions - usually the Hinduisms argue/claim that they are "the three aspects of the Holy Spirit", but this is absolutely false prophecy/witness.
What makes the Christian trinity monotheistic but the Hindu trinity polytheistic?
