Thus the Renaissance conception of humanitas had a two-fold aspect from the outset. The new interest in the human being was based both on a revival of the classical antithesis between humanitas and barbartias, or feritas, and on a survival of the mediaeval antithesis between humanitas and divinitas. When Marsilio Ficino defines man as a “rational soul participating in the intellect of God, but operating in a body,” he defines him as the one being that is both autonomous and finite. And Pico’s famous ‘speech’ ‘On the Dignity of Man’ is anything but a document of paganism. Pico says that God placed man in the center of the universe so that he might be conscious of where he stands, and therefore free to decide ‘where to turn.’ He does not say that man is the center of the universe, not even in the sense commonly attributed to the classical phrase, “man the measure of all things.”

It is from this ambivalent conception of humanitas that humanism was born. It is not so much a movement as an attitude which can be defined as the conviction of the dignity of man, based on both the insistence on human values (rationality and freedom) and the acceptance of human limitations (fallibility and frailty); from this two postulates result responsibility and tolerance.

…The humanist, then, rejects authority. But he respects tradition.

Erwin Panofsky, “The History of Art as a Humanistic Discipline" in Meaning in the Visual Arts

“Humanism- Most generally any philosophy concerned to emphasize human welfare and dignity, and optimistic about the powers of unaided human understanding.”
Simon Blackburn, The Oxford Dictionary of Philosophy

Blackburn goes on to discuss the Renaissance, but not well. He’s wrong on the history, but then history isn’t really his concern. Panofsky was a historian and a secularist. And he was a cosmopolitan. Blackburn is not.