Which passeth all understanding (ἡ ὑπερέχουσα πάντα νοῦν).
Either, which passes all power of comprehension, compare Eph 3:20; or, better, which surpasses every (human) reason, in its power to relieve anxiety. Compare Mat 6:31, Mat 6:32. For understanding, see on Rom 7:23.
Shall keep (φρουρήσει)
Lit., guard, as Rev., or mount guard over. God's peace, like a sentinel, patrols before the heart. Compare Tennyson:
“Love is and was my King and Lord,
And will be, though as yet I keep
Within his court on earth, and sleep
Encompassed by his faithful guard,
And hear at times a sentinel
Who moves about from place to place,
And whispers to the worlds of space,
In the deep night, that all is well.”
“In Memoriam.”
Gurnall, a little differently: “The peace of God is said to garrison the believer's heart and mind. He is surrounded with such blessed privileges that he is as safe as one in an impregnable castle” (“Christian in Complete Armor,” p. 419).
Hearts - minds (καρδίας - νοήματα)
For hearts, see on Rom 1:21. For minds, Rev., thoughts, see on 2Co 3:14. The guardianship is over the source and the issues of thought and will. “Your hearts and their fruits” (Alford).