Meaning: Truth is One, sages call it by different names. (God, Brahman, Allah, Isvara, Jesus, etc., are one). Hindus understand that the same god is called by different names and worshiped in different ways.

Vedas say that the Brahman, the formless god, is beyond all forms and is identified with different forms to make him accessible to human comprehension. So all those names and forms are of the same god when they are worshiped as god. That explains that, for a Hindu, every form of god (including the formless) that other religions worship is the same god they worship. A Hindu never thinks to spread his religion as he already sees unity in diversity.

Hinduism teaches that man’s true nature is Brahman (God) and his goal in this world is to realize his true nature (realizing he’s god). The entire Hindu idea is to guide the person to attain this god/self realization via different paths. Hinduism does not focus on itself (the religion), rather it focuses on guiding a person to rid himself of all material things (including religion) and reach the Brahman. Hinduism focuses on god/self. It does not focus on religion as an entity. It never tries or wants to spread itself.

To put it simply, Sanatana Dharma (Hinduism) does not focus on its own growth. It only focuses on the spiritual growth of mankind.

P.S. Swami Vivekananda’s all five speeches about Hinduism (together about 30-35 minutes) at the Parliament of the World’s Religions, Chicago 1893 give a detailed and beautiful answer to this question

I will quote the answer given by MaxMuller when a German Missionary asked the same question:

For Hindus, Religion is like mother.She is the most beautiful of all. If you try to prove your worth by propagating the beauty of your mother,then the outside world will see her as a prostitute.Hence propagate the value of your mother by the deeds you do called “Karma”. The value of the mother is known by the deeds done by the child. If your Karma is good, then automatically your mother will be most respected.