These words naturally lead us to consider two things:
I. The joy of being at rest. There are several kinds of rest which may indeed be subjects of thankfulness and gladness. There is the rest from enemies without; there is the rest from passions within; and there is the eternal rest of heaven. But the kind of rest of which we think today is the rest from doubt,doubt especially as to what it is needful to believe and to do if we seek to get to heaven. The very idea of rest implies something onwhich to rest; that is, it implies something above and beyond ourselves: it proves that in and of ourselves we can never have rest. Moses, speaking to the children of Israel, says, "Ye shall not do as we do here this day: every man that which is right in his own eyes." And why not? "For ye are not come unto the rest and the land which the Lord your God giveth you." No man has any more right to believe what he likes than to do what he likes; there is but one thing every one ought to do, which is right, and but one thing every one ought to believe, which is truth: and a man will as surely be punished for believing wrong as he will be for doing wrong.
II. But how can we believe? you will ask. And that brings us to our second head; namely, that we must be at rest before we can reach "the haven where we would be." In other words, unless we believe rightly, we shall never enter into heaven. No man can live as he ought without believing as he ought. Our Saviour, Christ, has promised this. "If any man," He says, "will do His will, he shall know of the doctrine whether it be of God." Therefore it follows that no man who believes wrong can be living right.
J. M. Neale, Sermons on Passages of the Psalms,p. 226.
Reference: Psalms 109:7. J. E. Vaux, Sermon Notes, p. 94.