Faith according to our Lord’s teaching in this paragraph, is primarily thinking; and the whole trouble with a man of little faith is that he does not think. He allows circumstances to bludgeon him. … We must spend more time in studying our Lord’s lessons in observation and deduction. The Bible is full of logic, and we must never think of faith as something purely mystical. We do not just sit down in an armchair and expect marvelous things to happen to us. That is not Christian faith. Christian faith is essentially thinking. Look at the birds, think about them, draw your deductions. Look at the grass, look at the lilies of the field, consider them. … Faith, if you like, can be defined like this: It is a man insisting upon thinking when everything seems determined to bludgeon and knock him down in an intellectual sense. The trouble with the person of little faith is that, instead of controlling his own thought, his thought is being controlled by something else, and, as we put it, he goes round and round in circles. That is the essence of worry. … That is not thought; that is the absence of thought, a failure to think.
Martin Lloyd Jones, Studies in the Sermon (Matthew 6:30)