In 1 Thessalonians Paul exhorts his readers to rejoice always and pray without ceasing and to give thanks in all circumstances! Those are pretty big agendas for the common person. In Philippians 4:4 Paul exhorts us all to “Rejoice always.” But then he goes on and says, “I will say it again; rejoice always!” As in the letter to the Thessalonians Paul moves from rejoicing to prayer. Why? 
Well, in the world we live in it’s impossible to be joyful in our circumstances all the time. There are many pains and losses and hurts and hardships that come our way and as we focus on them they will literally eat away our joy. But if we learn to entrust these things to God, and be thankful for His love and grace in our lives, we can find that it’s not all that hard to rejoice in the Lord. We not only find joy but God blesses us with the second fruit of the Spirit; Peace! Not just any ‘peace’ but a peace beyond our imagination.
Philippians 4:6-7 says, “Do not be anxious about anything, but in everything by prayer and supplication with thanksgiving let your requests be made known to God. And the peace of God, which surpasses all understanding, will guard your hearts and your minds in Christ Jesus.” Before I became a Christian I looked at Christians as poor, discontented and unhappy people. Some Christians gave me cause to think that! But I didn’t know what a carefree and secure life I was called to in becoming a Christian. While others around us struggle with all their individual projects, burdens, pains, hurts, sorrows, and the reality of death, we should be free from all such pressures. We shouldn’t be worried about what the future holds or what might happen. All the bases have been covered. We are certain about one thing; all will be well! Robert Leighton says of believers, “They have no care except for the care of love and how to please and honor their Lord. And for the skill and the strength they need, they also depend on God. From this springs unimaginable peace.”
The word guard was a military term that described a Roman soldier holding his weapon, walking back and forth in front of an open gate so that no one could enter. Paul is saying that this is how God will guard your hearts and give you His peace. Our Father is a Father who delights to give good gifts to His children. He will actively guard your heart against whatever is troubling it. He will not allow any menacing worry to enter. Paul says that peace will guard our hearts! MacArthur points out that, “John Bunyan’s allegory The Holy War illustrates how this peace guards the believer’s heart from anxiety, doubt, fear, and distress. In it Mr. God’s-Peace was appointed to guard the city of Mansoul. As long as Mr. God’s-Peace ruled, Mansoul enjoyed harmony, happiness, joy, and health.”