Finally, my brothers, be strong in the Lord, and in the power of his might.
This strength is needed under all the burdens, in all the conflicts and temptations of life, beneath its sorrows and its cares - strength of heart, strength of purpose, strength of will.
I. "BE STRONG." This is a strange command, just as strange as it would be for a physician to say to a weak man, "Be strong." It is like the command, "Rejoice in the Lord;" but it seems more difficult by any volition of our own to add to our strength than to add to our joy. Yet, as we can do much to regulate our emotions by determining what set of thoughts shall engage us, we can equally provide for an increase in our strength by a direct recourse to the secret and source of it. Our obedience to this command stands on the same footing as our obedience to God's other commandments; and if we continue to be weak, it is more than our misfortune, it is our fault. But there is nothing strange when we consider the secret of the origin of this strength. We are conscious of a sense of feebleness, of heartlessness, of hopelessness, which of itself goes far to disqualify us for duty, and gives us up an easy prey to the adversary of souls. It is to meet this want that God reveals himself to us as the great Giver of strength.
II. "BE STRONG IN THE LORD, AND IN THE POWER OF HIS MIGHT." The strength poured into us is strength in Christ, sprinting out of a realizing apprehension of the continued presence, love, and help of the Redeemer. "My strength shall be made perfect in weakness." A fly is able to walk upon the ceiling of a room. The cause is to be found in the vacuum in its webbed foot caused by its very weight, and it is thus enabled to hold on by the smooth surface of the ceiling. So our safety lies likewise in our emptiness. The soldier fights with greater confidence when he is led by a general who has been always successful. Wellington calculated the presence of Bonaparte at the head of an army as equal to a hundred thousand additional bayonets. Thus we understand the invincibility of the French army under his leadership. Thus the Christian fights with greater resolution because Christ is the Captain of his salvation.
III. THE COMMAND IMPLIES A CONTINUOUS DEPENDENCE UPON THE LORD. The strength is not given at once and in full measure, but according to the desire, the capacity, the faith, the need, the duty, the trial. Our lowest powers, those of the body, we get by growth, and they grow by exercise. Such is the law of our physical childhood, and no other is the law of our spiritual being. The sense of weakness obliges us to repair every day afresh to him for fresh supplies. "He giveth power to the faint; to them that have no might he increaseth strength." - T.C.
Parallel VersesKJV: Finally, my brethren, be strong in the Lord, and in the power of his might.