The steadfast are steady. Ballast is vital in sailing, providing stability in rough seas. Steadfastness of mind is like sturdy strength in a boat’s hull. Such balance comes from trust in God. Inner peace is an emotion resulting from a mental action. 

As you contemplate God’s love and faithfulness with your “head,” your “heart” responds with peace. Paul writes that as we are stabilized on Christ, “we are no longer to be children, tossed here and there by waves and carried about by every wind of doctrine…” (Ephesians 4:14 NASU)