From “Growing in the character of a disciple”: Chapter 1 – What it means to be a disciple and how Christian character is formed
The battle or conflict that we have to face is not only external to us, involving other people and demons. It is also an internal battle between our ‘old self’, otherwise known as our ‘old man’, ‘carnal nature’ or ‘flesh’, and our ‘new self’, ‘new man’ or ‘new nature’. That is our human spirit which is reborn within us when we become a Christian.
This aspect of our overall battle is like a civil war between the two very different natures that are both alive within us. They remain within us until we die. This part of our struggle means that we have to choose to do what our new nature wants and to refuse to do what our flesh or old self wants. See how apostle Paul puts it:
5 For those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit set their minds on the things of the Spirit. 6 For to set the mind on the flesh is death, but to set the mind on the Spirit is life and peace. 7For the mind that is set on the flesh is hostile to God, for it does not submit to God’s law; indeed, it cannot. 8 Those who are in the flesh cannot please God. 9 You, however, are not in the flesh but in the Spirit, if in fact the Spirit of God dwells in you. Anyone who does not have the Spirit of Christ does not belong to him. 10 But if Christ is in you, although the body is dead because of sin, the Spirit is life because of righteousness. 11If the Spirit of him who raised Jesus from the dead dwells in you, he who raised Christ Jesus from the dead will also give life to your mortal bodies through his Spirit who dwells in you.Romans 8:5-11 (ESV)
5 Put to death therefore what is earthly in you: sexual immorality, impurity, passion, evil desire, and covetousness, which is idolatry. 6 On account of these the wrath of God is coming. 7 In these you too once walked, when you were living in them. 8 But now you must put them all away: anger, wrath, malice, slander, and obscene talk from your mouth. 9 Do not lie to one another, seeing that you have put off the old self with its practices 10 and have put on the new self, which is being renewed in knowledge after the image of its creator. 11 Here there is not Greek and Jew, circumcised and uncircumcised, barbarian, Scythian, slave, free; but Christ is all, and in all. 12 Put on then, as God’s chosen ones, holy and beloved, compassionate hearts, kindness, humility, meekness, and patience, 13 bearing with one another and, if one has a complaint against another, forgiving each other; as the Lord has forgiven you, so you also must forgive. 14 And above all these put on love, which binds everything together in perfect harmony.Colossians 3:5-14 (ESV)
This first part of our battle, which is against our own sinful, flesh nature, is usually our greatest and hardest struggle. We have an opponent who is actually living inside us. It’s not a demon. It’s part of you. It operates as a kind of traitor or fifth-columnist seeking to undermine you from within and lead you astray. See Book Seven for a fuller discussion of our battle against the flesh. Now let us look more closely at the second and third battles that we have to fight. These are against the world system and also against the Devil and his demons.