Truth does not pay homage to any society, ancient or modern. Society has to pay homage to Truth or die. Societies should be moulded upon truth, and truth has not to adjust itself to society. (CWSV, vol. II, p. 84)
For example: If someone does not know or refuses to accept gravitational force, then he is not going to be spared the hurt if he jumps from a tree. Gravitational force cannot adjust to the desires, denials, foolishness or ignorance of a person, a person has to adjust oneself, align oneself with the truth or get perished. The human life, human society has come to such a state that unless it aligns itself with the truth it would perish. The grandest truth which humanity has to take into account is the Vedantic Truth of Oneness of existence. The more Swamiji moved in the West the more did he realize how the Vedantic truth is the need of the hour for the good of humanity.
He realized that, finding out this Oneness was the purpose of his life. He wrote,
Then you see, to put the Hindu ideas into English …is a task only those can understand who have attempted it. The dry, abstract Advaita must become living — poetic — in everyday life; out of hopelessly intricate mythology must come concrete moral forms; and out of bewildering Yogism must come the most scientific and practical psychology — and all this must be put in a form so that a child may grasp it. That is my life's work. (CWSV, vol.V pp.104-105)