No better admonition could have been given than that which the Apostle Paul gave to the Philippians: Finally, brothers, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable if anything is excellent or praiseworthy -think about such things (Philippians 4:8)
Paul mentions in 1 Corinthians 16:13: Be on your guard; stand firm in the faith; be men of courage; be strong. There must be spiritual development of strength in the body of Christ, as presented in God's Word (Ephesians 4:13-16).
Paul used this theme in his final writing to the Philippians; but he was not alone in this! The Wise Man in Proverbs also cautioned: Above all else, guard your heart, for it is the wellspring of life... As he thinks within himself, so is he. (Proverbs 4:23; Proverbs 23:7)
Paul taught that every thought of the heart could be brought into obedience to Christ (2 Corinthians 10:5) and thus he urged the saints (Christians) to think on those things that would elevate and sanctify them. To purify our hearts, we should remember that God through His Word judges the thoughts and attitudes of the heart. (Hebrews 4:12).
It becomes much easier to think on the proper things when we realize that God probes our heart and its purpose!