James 4:15

For that ye ought to say - Instead of what you do say, "we will go into such a city," you ought rather to recognise your absolute dependence on God, and feel that life and success are subject to his will. The meaning is not that we ought always to be saying that in so many words, for this might become a mere ostentatious form, offensive by constant unmeaning repetition; but we are, in the proper way, to recognise our dependence on him, and to form all our plans with reference to his will.

If the Lord will ... - This is proper, because we are wholly dependent on him for life, and as dependent on him for success. He alone can keep us, and he only can make our plans prosperous. In a thousand ways he can thwart our best-laid schemes, for all things are under his control. We need not travel far in life to see how completely all that we have is in the hands of God, or to learn how easily he can frustrate us if he pleases. There is nothing on which the success of our plans depends over which we have absolute control; there is nothing, therefore, on which we can base the assurance of success but his favor.