KEY THOT: There are two principles at work in our bodies: flesh vs. Spirit. The flesh refers to the old principle of sin that has been wired into our brain and body because of pre-Christian thought-patterns and mindsets. The "flesh" is not just a reference to the soft tissue of the body but also to all the sinful habits that are enfleshed in it. But when we accepted Jesus as our Lord and Saviour, the Spirit of God came into our bodies and dwells within our body: "do you not know that your body is a temple of the Holy Spirit within you, whom you have from God?" (1 Corinthians 6:19). Before we knew Christ, we only have one principle at work in our body, which is sin: "the body is dead because of sin" (v.10). But now that we are in Christ, another principle is at work--the Spirit of life now dwells within us in the same body.
So now we have a daily choice between two principles. We can set our minds on the things of the flesh which will lead to spiritual death: "To set the mind on the flesh is death" (v.6a). Or we can set our minds on the things of the Spirit which will lead to spiritual life: "but to set the mind on the Spirit is life and peace" (v.6b).
But what are the "things of the flesh"? According to Galatians 5:19-21, the flesh is defined by the following activities: "Now the works of the flesh are evident: sexual immorality, impurity, sensuality, idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions, divisions, envy, drunkenness, orgies, and things like these." So, if we are still indulging in these things, we will slowly die spiritually. In fact, if we do not repent and continue to participate in these activities as believers, the death principle will eventually overcome the life principle. That is why Paul repeated his warning he gave to the Galatian believers: "I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God" (Galatians 5:21). Note that he is not referring to unbelievers but to believers who already have the "deposit of the Spirit" which guarantees their inheritance in the kingdom of God.
An analogy might help. Suppose we buy a life insurance. When we make the first premium payment, we are guaranteed that if we die, we shall inherit the full endowment promised. But in order that we will inherit the full payment, we must continue to pay our regular premiums. If we renegade on our premiums, we may receive reminders to pay up. But if we choose to ignore these reminders and fail to pay our premiums, the insurance will lapse and we will not receive the full endowment promised.
So, it is with our eternal life insurance: God has given us the Spirit within us as our first premium: "Having believed, you were marked in him with a seal, the promised Holy Spirit, who is a deposit guaranteeing our inheritance until the redemption of those who are God's possession" (Ephesians 1:13-14). It's entirely grace--we didn't do anything to receive our first premium. But in order to keep our eternal life guarantee "live", we must continue to set our minds on the "things of the Spirit" by walking in the righteousness of Christ: "But if Christ is in you, although the body is dead because of sin, the Spirit is life because of righteousness" (v.10).
So, as we walk by the Spirit, God continues to graciously pour out His Spirit upon us and our spiritual premiums grow within us. This will strengthen our assurance of inheriting the kingdom as our lives are slowly becoming full of the Spirit of God, bearing the fruit of the Spirit.
Those who think that grace means they can sin as they please and indulge in the "things of the flesh" are setting their lives on the path of death, not life. They may ultimately fail to inherit the kingdom when they have finally lost the deposit (Holy Spirit) that guarantees the inheritance by continually quenching and grieving the Spirit.
Father, teach us to live by Your Spirit and reject the deeds of the flesh, so that we might inherit the kingdom because You have already given us the deposit of the Spirit of God as the guarantee. Amen.