Charles Haddon Spurgeon expressed truth about the two natures within us much more eloquently than I can. I shall include two of his devotionals dealing with the issue.
Spurgeon on Galatians 5:17: For the flesh lusteth against the Spirit, and the Spirit against the flesh.
In every believer*s heart there is a constant struggle between the old nature and the new. The old nature is very active, and loses no opportunity of plying all the weapons of its deadly armoury against newborn grace; while on the other hand, the new nature is ever on the watch to resist and destroy its enemy. Grace within us will employ prayer, and faith, and hope, and love, to cast out the evil; it takes unto it the *whole armour of God,* and wrestles earnestly. These two opposing natures will never cease to struggle so long as we are in this world. The battle of *Christian* with *Apollyon* lasted three hours, but the battle of Christian with himself lasted all the way from the Wicket Gate in the river Jordan. The enemy is so securely entrenched within us that he can never be driven out while we are in this body: but although we are closely beset, and often in sore conflict, we have an Almighty helper, even Jesus, the Captain of our salvation, who is ever with us, and who assures us that we shall eventually come off more than conquerors through Him. With such assistance the new-born nature is more than a match for its foes. Are you fighting with the adversary to-day? Are Satan, the world, and the flesh, all against you? Be not discouraged nor dismayed. Fight on! For God Himself is with you; Jehovah Nissi is your banner, and Jehovah Rophi is the healer of your wounds. Fear not, you shall overcome, for who can defeat Omnipotence? Fight on, *looking unto Jesus*; and though long and stern be the conflict, sweet will be the victory, and glorious the promised reward.
*From strength to strength go on;
Wrestle, and fight, and pray,
Tread all the powers of darkness down,
And win the well-fought day.*
Spurgeon on Genesis 1:4: And God divided the light from the darkness.
A believer has two principles at work within him. In his natural estate he was subject to one principle only, which was darkness; now light has entered, and the two principles disagree. Mark the apostle Paul*s words in the seventh chapter of Romans: I find then a law, that, when I would do good, evil is present with me. For I delight in the law of God after the inward man: but I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin, which is in my members.* How is this state of things occasioned? The Lord divided the light from the darkness.* Darkness, by itself, is quiet and undisturbed, but when the Lord sends in light, there is a conflict, for the one is in opposition to the other: a conflict which will never cease till the believer is altogether light in the Lord. If there be a division within the individual Christian, there is certain to be a division without. So soon as the Lord gives to any man light, he proceeds to separate himself from the darkness around; he secedes from a merely worldly religion of outward ceremonial, for nothing short of the gospel of Christ will now satisfy him, and he withdraws himself from worldly society and frivolous amusements, and seeks the company of the saints, for *We know we have passed from death unto life, because we love the brethren.* The light gathers to itself, and the darkness to itself. What God has divided, let us never try to unite, but as Christ went without the camp, bearing His reproach, so let us come out from the ungodly, and be a peculiar people. He was holy, harmless, undefiled, separate from sinners; and, as He was, so we are to be nonconformists to the world, dissenting from all sin, and distinguished from the rest of mankind by our likeness to our Master.