"Be holy both in body and in spirit" (1 Cor. 7:34).
"Let us cleanse ourselves from all defilement of flesh and of spirit" (2 Cor. 7:1).
If we want to walk according to spirit, we should always keep our spirit holy. An unholy spirit will mislead people. Having improper thoughts in criticizing people or conjecturing about matters, remembering others’ sins, lacking love, talking too much, criticizing harshly, and being self-righteous, unreceptive towards exhortation, jealous towards the brothers, conceited, and so forth are all able to defile the spirit. An unholy spirit cannot be new and fresh.
In our pursuit of a spiritual living, we cannot ignore a sin even for a minute. Sins damage us more than anything else. Even though we understand how to be free from sins and how to walk according to spirit, we still need to be careful that we do not unconsciously return to our past sinful situation. When sin comes, it is impossible to walk according to the spirit. We must always be watchful to have the attitude of being dead so that sin will not overcome us or enter into our spirit to poison it. Without holiness, no man can see the Lord.
"Became strong in spirit" (Luke 1:80).
Our spirit must gradually grow and become strong. This is indispensable in spiritual living. Many times we feel that our spirit is not strong enough to control our soul and body, especially when our soul is stimulated or when our body is weak. Sometimes when we want to help others, seeing the heavy burden in their spirit, we feel how powerless our own spirit is. We are unable to release them. Sometimes in our battle with the enemy, we see that we are not strong enough in our spiritual strength, finding it difficult to "wrestle" unto victory with the enemy for a long time. Many times we really feel that our spiritual strength is not enough to control everything. There are many areas in our living and work which we can barely manage. How we wish that we had a stronger spirit!
Once the spirit is strong, there is a sharp intuition and discerning power. There is also the ability to refuse everything which does not belong to the spirit. Some believers intend to walk according to the spirit, but they are unable because the power in their spirit is not sufficient to control everything; on the contrary, it is subject to being controlled. We cannot expect the Holy Spirit to replace us in doing all the work; our regenerated spirit needs to co-labor with the Holy Spirit. We must learn how to use our spirit and use it as best we know how. If a believer uses his spirit, it will gradually become strong and have the power to break through all that is hindering the Holy Spirit, regardless of whether it is the stubborn will, the confused mind, or the unrestrained emotion.
The Bible tells us that the spirit can be wounded (Prov. 18:14), meaning that the spirit can feel hurt. A wounded spirit is very weak. If our spirit is strong, we are able to stand the stimulation of the soul and not be shaken. Moses’ spirit may be considered as being very strong, but because he did not constantly keep it strong, the Israelites were able to provoke his spirit unto sinning (Psa. 106:33). If our spirit is strong, we will be able to declare victory in the Lord whatever the situation and however great the suffering in the body or sorrow in the emotions.
Only the Holy Spirit can grant us the strength needed by the inner man. The strength in our spirit comes from the power of the Holy Spirit. Yet there is also a need for the spirit itself to be trained. After a believer has learned how to walk according to spirit, he will learn how to use the power of his spirit in his work and not his natural power. In life, he will know how to live by the spiritual life and not rely on the soulish life. In battle, he will learn how to use the strength in the spirit and not his soulish strength to resist, attack, and oppose Satan and his evil spirits. This naturally will improve and needs to improve. As a believer walks according to the spirit, he receives more power from the Holy Spirit. At the same time, his spirit becomes stronger. A believer should always keep his spirit in a strong condition. He should never let it lose its power lest it become unable to deal with any needs when they arise.
(Spiritual Man, The (3 volume set), Chapter 25, by Watchman Nee)