Be of good courage, and He shall strengthen your heart, all ye that hope in the LORD. ~ Psalm 31:24

The text says, “Be of good courage, and He shall strengthen your heart;” wherein it is implied that, sometimes, the heart of them that hope in the Lord grows weak. As you well know, heart disease is a very dangerous disease; even if a very little is wrong with the heart, it is a serious matter, for every other part of the body will be affected. Some of God’s own people are occasionally, and many of them very often, subject to a weakness of the heart. They lose their courage, their joy departs from them, and they become timorous and fearful.

Sometimes we have seen those who hope in the Lord very weak in heart under great suffering. Pain follows pain; it seems as if every cut of the knife went deeper than the last, and that the knife was sharper every time… At such times, the spirits sink, and the heart’s action grows feebler and feebler…I do not think that we are at all lacking in confidence in ourselves,—at any rate, some of us are not; but it is confidence in God which is wanted, and that is quite another thing. This confidence makes the feeble strong, and the timid brave; may we all have a large share of it! God deliver us from faintness of heart, lest we injure the kingdom of our Lord by withholding our service! ~ C.H. Spurgeon

http://www.spurgeon.org/sermons/2455.htm