Washington in his Farewell Address
said "Whatever may be conceded to the influence of refined education on minds of peculiar structure, reason and experience both forbid us to expect that national morality can prevail in exclusion of religious principle
That is, in Washington's view, without religious principle, American people cannot be united as one.
Similarly, Einstein once said "Science without religion is lame, religion without science is blind."
But Einstein was never religious, for he denied a personal God. His religion is a devotion to science.
So what is the religious principle mentioned by Washington? He referred to "the benign Parent of the Human Race" in his first inaugural address. Yet he didn't mention God in his second inaugural address at all. It is very unusual for a President of United States to omit His Almighty in a presidential inauguration. It is reasonable to guess that his religious principle is a religion-like devotion to the Constitution, which had the place of God in his second inaugural address.
Thus, both Washington's religious principle and Einstein's principle of religion may be one thing. This one thing is for the happiness of humankind.
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