The Thanksgiving of the receiver incites the Giver to give gifts greater than the first. He that returns no thanks in small matters is a dissembler and dishonest in greater ones also. If a man is ill and he recognizes his ailment, his healing will be easy. If he confesses his pain, he draws nigh its cure. There are many pangs for the unyielding heart, and the patient who resists his physician amplifies his torment. There is no unpardonable sin, save the unrepented one. Nor does any gift remain without addition, save that whites received without thanksgiving.

The Ascetical Homilies of Saint Isaac the Syrian

Homily Two : On Thankfulness to God, In Which There Are Also Essential Elementary Lessons