Eph. 4:22 Deleting the old man

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Ephesians 4 22-24 That ye put off concerning the former conversation the old man, which is corrupt according to the deceitful lusts;
And be renewed in the spirit of your mind;
And that ye put on the new man, which after God is created in righteousness and true holiness.

Just because you have gained a victory over your lower nature, it does not mean that your triumph will last forever. The situation can be compared to a country which has forced the enemy into retreat: it cannot be sure that they will not renew their attacks. 
For, while the conqueror is resting on their laurels, the vanquished are planning their revenge. 

The same thing applies to your lower nature: even if you have managed on occasion to conquer a few of its manifestations – bad thoughts, bad feelings – you will never be sure of having permanently triumphed over it. 
From one moment to the next, it can rebel and there you are, defeated. 
But if you always stay in touch with your higher nature, instead of being like a winner who is never permanently assured of their victory, you have a powerful ally on whom you can always count. 
And even if sometimes you are a little tired, this ally continues to hold your lower nature in check.