What, according to O'Sullivan, is the immutable truth and benefience of God?

I assume you are referring to O'Sullivan's article "The
Great Nation of Futurity."  If so, O'Sullivan argues that the "immutable truth and
beneficence of God" is that all people should be free and should live under a good
government and in a good society like that of the United States.  This is why, he says,
the US is destined to expand its territory and create a country that will be a beacon to
the world.

As O'Sullivan says, America is based on the
ideas of

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...freedom of conscience, freedom of person,
freedom of trade and business pursuits, universality of freedom and
equality.

He argues that the
work of the United States is to spread these ideas as far as possible.  By spreading
these ideas and making more people live according to them, the US will be fulfilling its
mission.  When it brings those things to the people of the world, it will be bringing
them the "immutable truth and beneficence of God."