Trust not in oppression - The general meaning here is, that we are not to trust in anything but God. In the previous verse the psalmist had stated reasons why we should not trust in men of any rank. In this verse he enumerates several things on which people are accustomed to rely, or in which they place confidence, and he says that we should put no confidence in them in respect to the help which we need, or the great objects which are to be accomplished by us. The first thing mentioned is oppression; and the idea is, that we must not hope to accomplish our object by oppressing others; extorting their property or their service; making them by force subject to us, and subservient to our wishes. Many do this. Conquerors do it. Tyrants do it. The owners of slaves do it.
And become not vain in robbery - That is, Do not resort to theft or robbery, and depend on that for what is needed in life. Many do. The great robbers of the world - conquerors - have done it. Thieves and burglars do it. People who seek to defraud others of their earnings do it. They who withhold wages from laborers, and they who cheat in trade, do it.
If riches increase, set not your heart upon them - If you become rich without oppression, or without robbery. If your riches seem to grow of themselves - for that is the meaning of the original word (compare Mark 4:2) - do not rely on them as being all that you require. People are prone to do this. The rich man confides in his wealth, and supposes that he has all he needs. The psalmist says that none of these things constitute the true reliance of man. None of them can supply his real needs; none can defend him in the great perils of his existence; hone can save his soul. He needs, over and above all these, a God and Saviour; and it is such a God and Saviour only that can meet the real needs of his nature.