If someone is judged worthy to receive the gift of knowledge but allows his heart to be full of bitterness or rancor or aversion to another, it is as if he had been struck in the eye by a thornbush. That is why knowledge is no good without charity.
Understand me. It is so simple. People who do not know God, or who go against Him, are to be pitied; the heart sorrows for them and the eye weeps. Both paradise and torment are clearly visible to us: We know this through the Holy Spirit. And did not the Lord Himself say, 'The kingdom of God is within you?' Thus eternal life has its beginning here in this life; and it is here that we sow the seeds of eternal torment.
If you have received from God the gift of knowledge, however limited, beware of neglecting charity and temperance. They are virtues which radically purify the soul from passions and so open the way of knowledge continually.