"But grow in grace, and in the knowledge of our Lord and Savior Jesus Christ." 2 Peter 3:18
Growth is the sure mark of life. We see this in vegetation, in the animal creation, in the growth of our own bodies, and of every other thing in which there is life. Where, then, there is the life of God in the soul, there will be a growth in that life. Paul says to the Thessalonian Church--"We are bound to thank God always for you, brethren, as it is fit, because your faith grows exceedingly" (2 Thess. 1:3); and Peter says, "But grow in grace, and in the knowledge of our Lord and Savior Jesus Christ." There is "an increasing in the knowledge of God" (Col. 1:10), and "a coming in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fullness of Christ" (Eph. 4:13). It was for this increasing knowledge of the Son of God that Paul stretched every desire of his soul when he followed after, if that he might apprehend that for which also he was apprehended of Christ Jesus; and thus reaching forth unto those things which were before, he pressed toward the mark, for the prize of the high calling of God in Christ Jesus (Phil. 3:12-14).
This is not what is called 'progressive sanctification', as if the flesh got holier and holier, for that is still ever "the old man, which is corrupt according to the deceitful lusts;" but this is a growth of that "new man, which after God is created in righteousness and true holiness." After this growth in grace, this closer conformity to the image of Christ, should we ever be striving with all the powers of our soul; not satisfied with a low and lean state before God, but with unceasing prayer and supplication, begging of the Lord that we might be "filled with the knowledge of his will in all wisdom and spiritual understanding, that we might walk worthy of the Lord, unto all pleasing, being fruitful in every good work, and increasing in the knowledge of God" (Colossians 1:9, 10).