In the first stanza of "Man," the speaker claims that no one would built a stately (grand or impressive) house unless he intended to live there. Since God created the Earth and "man" (humans), God must have intended to live there: in the heart and consciousness of each person and/or as a spiritual presence in the world.
In the second stanza, the speaker notes that man (humanity) is this stately house because man is everything:
For man is ev'ry thing,And more; he is a tree, yet bears more fruit;A beast, yet is, or should be, more;Reason and speech we only bring;Parrots may thank us if they are not mute,They go upon the score.
O mighty love! Man is one world, and hathAnother to attend him.