Why did Jesus say to His disciples, "Let not your heart be troubled"?
He said this in order to console them beforehand, so that they might not be downcast, or waver in their faith, in case the predicted tribulations should actually come upon Him and them. Thus it is that God never fails to comfort those who suffer persecution for His sake.

Prayer to Saint Philip
O Saint Philip, chosen disciple of the Lord, who brought Nathaniel to Christ, who most zealously preached thy Lord, Jesus Christ, and out of love to Him willingly gave thyself to be nailed to the cross, and put to death, obtain, I beseech thee, for me, and for all men, grace with zeal to bring others to the practice of good works, to have a great desire after God and His truths, and, in hope of the eternal blissful contemplation of God, to bear patiently the adversities and miseries of this life. Amen.

Prayer to Saint James
O Saint James, who lived so temperately and strictly, who, like thy master, prayed so earnestly and constantly for thy tormentors, I beseech thee that thou wouldst procure us from Jesus grace, after thy example, to live sober and penitential lives, and to worship God in spirit and in truth. Obtain for us, therefore, the spirit with which thou didst write thine epistle, that we may follow thy doctrine, be diligent in good works, and, like thee, love and pray for our enemies. Amen.