John Chrysostom

"Take heed, brethren, lest there be in any of you an evil heart of unbelief in departing from the living God." For from hardness unbelief ariseth: and as in bodies, the parts that have become callous and hard do not yield to the hands of the physicians, so also souls that are hardened yield not to the word of God. For it is probable besides that some even disbelieved as though the things which had been done were not true.

Therefore he says, "Take heed lest there be in any of you an evil heart of unbelief in departing From the living God." For since the argument from the future is not so persuasive as from the past, he reminds them of the history, in which they had wanted faith. For if your fathers (he says) because they did not hope as they ought to have hoped, suffered these things, much more will you. Since to them also is this word addressed: for, "To-day" (he says) is "ever," so long as the world lasts.

Above all things indeed, he says, "let there not be an evil heart of unbelief." But even suppose there should be, let no man despair, but let him recover himself; for as long as we are in this world, the "To-day" is in season.