From Scott Anderson at Desiring God:
Although he may overstate the case a little—thinking is not the essence of faith—he is right that thinking is essential to faith.
Faith, according to our Lord’s teaching in this paragraph [Matthew 6:25-34], 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.
That is the real difficulty in life. Life comes to us with a club in its hand and strikes us upon the head, and we become incapable of thought, helpless and defeated. The way to avoid that, according to our Lord, is to think. 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, and draw your deductions. Look at the grass, look at the lilies of the field, consider them.
The trouble with most people, however, is that they will not think. Instead of doing this, they sit down and ask, What is going to happen to me? What can I do? That is the absence of thought; it is surrender, it is defeat. Our Lord, here, is urging us to think, and to think in a Christian manner.
That is the every essence of faith. 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.
—Studies in the Sermon on the Mount, p. 399. Paragraphing added.