Words have power, but they should be backed the power of truth, not by lies and deceit. That singular truth being that each of us is just as human as the next, if not for the "walls" dividing us in the prior line. He is also specifically utilizing the language, and thus words, of the Empire that oppresses his people as a tool to appeal to them.

Tagore understood that the primary audience of an English translation would be Christian, and thus utilized an explicit reference to Christianity with a capitalized Father referring to the Christian God. He is utilizing both the language and religion of another people in support of his own. This would strengthen his call among that audience and helps turn this entire poem into a prayer and call to the divine.

This implies a sense of pride and confidence in what one was doing, which reinforces that the mind is without fear, because the body is visibly showing that. This also serves to highlight that the body and mind are acting in unison.

The clear stream represents progress as sustaining life, while holding fast to detrimental traditions, like the caste system, turns things into a desert devoid of life.

Free here can mean multiple things.

Free as in the monetary sense and free as in it is available to everyone without restriction. Both definitions feed into the next line by opening up knowledge outside of those it is typically restricted to, which is an exceptionally notable statement from Tagore, due to his standing at the very top of the traditional Hindu caste system. In his arguments here, he is explicitly calling for the dissemination of knowledge to everyone.

Tagore defines the mind in What is Art? very specifically as the logical and reasoning half of what might generally be considered the whole mind by a reader. Emotions, like fear, are found within the personal man, or personality, instead of the physical man. Thus, if the mind is without fear, then this inner personality has been sated and exists in harmony with the physical mind.