In other words, the only doctrine of the religion is not to be doctrinaire?
Short answer is actually Yes and No
Yes answer explanation: The foundational aspects of Hinduism is based on universal principles and universal truth that governs all beings (including human beings). This is the assertion about the the Paramarthika Satyam which does not have its origin in human thought.
No answer explanation: The world we find ourselves in, is full of limitless variety and quality of living and non-living things . They appear, sustain & destroy each other in an ecosystem. In this world/universe, Vyavaharika Satyam (truth) sustains the world of opposites. The umbrella religion of Hinduism is unique in the sense that it acknowledges the universal aspects of this phenomenological world of variety and does not *impose* any commandments & absolutes on human beings (like Thou Shalt Not ..... etc). This is because the 'Samanya Dharma' is known to all human beings no matter where they are born. Individual traditions within Hinduism may impose rules on its followers but they do respect existence of other traditions. Obviously there have been fights between people representing various tradition but those are attributable to human weakness and humans not following principles of Dharma.
Our emphasis is therefore on following Swadharma (which has to be aligned with Samanya Dharma) and such an approach has allowed people of all kinds and faiths to co-exist in an interdependent manner.
For a Hindu, Isvara can be visualized in a christian cross or an empty room (as in Chidambara Rahasyam). Such an Isvara is both immanent and transcendent. This sense of respect and acceptance of each other is best illustrated by Swami Vivekananda's chicago address and its basis is the above understanding. To quote Swami Vivekananda -
" I am proud to belong to a religion which has taught the world both tolerance and universal acceptance. We believe not only in universal toleration, but we accept all religions as true. I am proud to belong to a nation which has sheltered the persecuted and the refugees of all religions and all nations of the earth. I am proud to tell you that we have gathered in our bosom the purest remnant of the Israelites, who came to Southern India and took refuge with us in the very year in which their holy temple was shattered to pieces by Roman tyranny. I am proud to belong to the religion which has sheltered and is still fostering the remnant of the grand Zoroastrian nation. I will quote to you, brethren, a few lines from a hymn which I remember to have repeated from my earliest boyhood, which is every day repeated by millions of human beings: "As the different streams having their sources in different paths which men take through different tendencies, various though they appear, crooked or straight, all lead to Thee.""
The last part is the famous line known to most people doing Sandhyavandanam
"आकाशात् पतितं तोयं यथा गच्छति सागरम् सर्व देव नमस्कारः केशवं प्रतिगच्छति"
This universal approach to relating to each other does not mean accepting Adharma of others. For example the theologies of Christianity and Islam includes the cancerous aspects of conversion of others at any cost. Hindus have no problem acknowledging that different people may feel exclusive about their swadharma or their Ishta Devata.
It is an issue when anyone wants to convert others to their unique belief system. Cancerous cells have to be fought and neutralized and that is Dharma for mutual sustenance. In this sense Hindus do not have a doctrine of accepting any belief system in this order of phenomenological world.
There are excellent verses about the importance of following Swadharma in B.Gita Chapter 3. This world view of mutual respect and universal acceptance is unique to Hinduism.
The no answer is about saying no to adharma and in that sense the doctrine is very specific and categoric.
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