In The Root of the Righteous, A.W. Tozer writes a chapter about the encroaching jungle. He points out that you can put all sorts of work into clearing the jungle and planting a field, but that if you leave that field to itself for just a couple of years, the jungle will overcome the cleared land and take over once again.
The same is true of the Christian life. A lot of sweat and tears, prayers and loss of sleep goes into sanctification. It is a mighty work of God to be made holy by the work of Christ Jesus our Lord. The surrendered life is a radical transformation compared to the sinful nature of the old self. But if the holy, sanctified, blameless Christian takes their eyes off the Lord for only a minute, the jungle of the flesh starts to grow back and take root in that life. Just as the field must be continually tended in order to bear fruit, such is the Christian life. It does not take much time away from the Lord at all for that life to return to the wild, jungle state.
Thus it is essential to be consistent in the Christian walk. “No branch can bear fruit by itself; it must remain in the vine. Neither can you bear fruit unless you remain in me. I am the vine; you are the branches. If a man remains in me and I in him, he will bear much fruit; apart from me you can do nothing” (John 15:4-5). To remain in Christ is to remain in the cleared, fruitful field; to depart from him is for you to allow the jungle to take over again and conquer your life. So abide in the Vine, and live. Apart from him, you can do nothing; you can bear no fruit in your life alone.