Consider first, that the life of a Christian is a perpetual warfare, according to that of holy Job, 'The life of man upon earth is a warfare,' Job vii. 1. His time of a true, settled, and solid peace, is not to come till after many a conflict, and many a victory. He must fight his way to heaven against a set of cruel, deceitful, and obstinate enemies, who will never let him alone, either in life or death. These are they of whom the apostle writes, Eph. vi. 11, 12, 'Put you on the armour of God, that you may be able to stand against the deceits of the devil. For our wrestling is not against flesh and blood, but against principalities and powers, against the rulers of the world of this darkness, against the spirits of wickedness in high places,'- that is against all the legions of demons who dwell in this air that surrounds us. These wicked spirits mortally hate us, because we are created to fill up the places they have forfeited by sin, and therefore they never cease to employ a thousand tricks and artifices, a thousand snares and allurements, to draw us away from God, into the pit of sin and damnation. But for our comfort their power is greatly restrained; they can employ no force against us, nor do us any real hurt at all, except it be by our own free consent, by our unhappily yielding ourselves to them. And on the other hand, we have a far greater strength engaged to fight on our side, not only of all the host of heaven, ever ready to guard and defend us, but even of the king of heaven himself, whose battles we are fighting, and who never forsakes his soldiers in the war, if they do not first desert from him. And what can the soldiers of Christ desire more? God is for us, what matter is it than who is against us? We are sure of victory if we do not fling down our arms and desert to the enemy.
Consider 2ndly, that he devil and all his wicked ones, knowing how little they can do against us of themselves, have unhappily engaged the world and the flesh to side with them as auxiliaries in this warfare. Upon these they depend more than upon all their own troops; and these, in effect, annoy us more, and commonly do much more mischief to our souls, than all those spirits of darkness. By the world we mean the whole collection of poor mortals who have been already deluded by Satan into the broad road of sin and perdition, and who have embraced, both in principles and practice, the wicked laws, maxims, and customs which he has introduced, and continually propagate them both by word and example. This is that world, the slave of Satan and the mortal enemy of the gospel of Christ, against which he so often pronounces his woes; this is that Babylon, the city of the devil, that is waging war against the people of God, and seeking to carry them away captives and to subject them to its wicked laws. This wicked world is made up of the lust of the flesh, the lust of the eyes, and the pride of life, 1 John ii. 19. This wicked world and all its pomps we have renounced at our baptism. Against this capital enemy of our souls we must fight till death.
Consider 3rdly, that if the world be a dangerous enemy to a Christian soul - because it requires a great courage and strength to go against the current of all its maxims and practices, and to despise both its allurements and its censures - the flesh, on the other hand, is still more difficult to overcome; because it is a domestic enemy that is always at hand, even within this very castle of ours, which is continually besieged by the devil and the world; 'tis a traitor that holds a perpetual correspondence with these enemies, and is ever ready to open the gates of the soul to them. For by the flesh we mean our own evil inclinations, passions, and lusts, which never cease to wage war against the Spirit, and which violently incline us to take part with the devil and the world against our God. Hence our Lord, in his gospel, not only requires that we should renounce all other things in the world, how near or dear soever they may be to us, to follow him; but more especially inculcates that we must renounce also ourselves, and hate ourselves in this life, if we desire to be his disciples and to save our souls. This, then, is the very chief part of the Christian's warfare - to fight continually against ourselves, and to spare no pains to subdue our own flesh - that is, all our corrupt inclinations and passions.
Conclude to declare an eternal war against all these three enemies, and to fight manfully against them till death, and thou shalt not fail to receive the crown of life.