Perhaps the greatest single failure in the Christian life is the refusal to give thanks. Thanks that is dependent upon success or the fulfillment and pleasure of our own will is indeed thanksgiving – but is weak indeed. It is easy to give thanks for our pleasures and self-satisfactions (though even then we often forget to give thanks).
Fr. Stephen Freeman (source)
You could say that thanksgiving to God in every circumstance is the mark of a Christian; it is our daily activity, the source of our joy, the fountain from which our hearts beat and our days become something more than rote and given. They become life and joy and full.
It is absurd and appears to be the height of foolishness to give thanks for the cancer, for the persecution, for the abuse, for the sufferings of all sort. It appears wrong, as those tyrannies have fled far from the will of God. Those horrors that afflict us (and we inflict on others) are blackness.
Yet we may give thanks. In spite of the darkness, in spite of our misery, as an act of faith and opposition to our Enemy.
As an act of faith in our God, who is Good, who is Love, whose ways are so far beyond that they are darkness to us, whose light is beyond light so that we cannot perceive, whose Love is beyond our fickle and stuttering affection.