17 “And confess to the remembrance of His holiness.” Now made glad, now rejoicing in the Lord, confess unto Him; for unless it were His will, you would not rejoice in Him. For the Lord Himself says: “These things I have spoken to you: that in Me ye might have peace. But in the world you shall have tribulation.” If you are Christians, look for tribulations in this world; look not for more peaceful and better times. Brethren, you deceive yourselves; what the Gospel does not promise you, promise not to yourselves.
You know what the Gospel says; we are speaking to Christians; we ought not to disobey the faith. The Gospel says this, that in the last times many evils, many stumbling-blocks, many tribulations, much iniquity, shall abound; but he that shall endure unto the end, the same shall be saved. “The love,” it says, “of many shall wax cold.” Whosoever then has been steadfastly fervent in spirit, as the Apostle says, “fervent in spirit,” his love shall not wax cold: because “the love of God is shed abroad in our hearts by the Holy Ghost, which is given unto us.” Let no man therefore promise himself what the Gospel does not promise.
Behold, happier times will come, and I am doing this, and purchasing this. It is good for you to listen to Him who is not deceived, nor has deceived any man, who promised you joy not here, but in Himself; and when all here has passed away, to hope that with Him you will for ever reign; lest when thou dost wish to reign here, you may neither enjoy gladness here, nor find it there.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)