In Old Testament times, gatekeepers guarded the temple to prevent those who were unclean from entering (2 Chronicles 23:19) — See also Porters and Doorkeeper (see another doorkeeper article).
In the Christian dispensation [age], Paul calls our bodies God’s “temple” (Romans 8:9-11; 1 Corinthians 3:16-17; 2 Corinthians 6:16). Many evil forces are at work attempting to assault God’s new dwelling (2 Corinthians 4:7-11).
Evil always strives to gain a foothold through the unfortified areas of our spiritual life — places where envy, strife, or divisions may undermine us (1 Corinthians 3:3; cf. Gal. 5:19-21; James 3:13-16).
Each of us must be on guard against the enemy of our souls and never give place to the devil (Ephesians 4:27; cf. 1 Peter 5:8-9).
Paul tells us in Philippians 4:8 what may enter our minds — whatever is true, noble, just, pure, lovely, of good report, virtuous, and praiseworthy. The resulting peace will guard the gate of our hearts and minds (Philippians 4:7).