Psalms 49:3

We seldom meet with a more solemn introduction: there is no truth of greater importance. Let all hear this with application to ourselves. The poor are in danger from undue desire toward the wealth of the world, as rich people from undue delight in it. The psalmist begins with applying it to himself, and that is the right method in which to treat of Divine things. Before he sets down the folly of carnal security, he lays down, from his own experience, the benefit and comfort of a holy, gracious security, which they enjoy who trust in God, and not in their worldly wealth. In the day of judgment, the iniquity of our heels, or of our steps, our past sins, will compass us. In those days, worldly, wicked people will be afraid; but wherefore should a man fear death who has God with him?

My mouth shall speak of wisdom. Inspired and therefore lifted beyond himself, the prophet is not praising his own attainments, but extolling the divine Spirit which spoke in him. He knew that the Spirit of truth and wisdom spoke through him. He who is not sure that his matter is good has no right to ask a hearing. And the meditation of my heart shall be of understanding. The same Spirit who made the ancient seers eloquent, also made them thoughtful. The help of the Holy Ghost was never meant to supersede the use of our own mental powers. The Holy Spirit does not make us speak as Balaam's ass, which merely uttered sounds, but never meditated; but he first leads us to consider and reflect, and then he gives us the tongue of fire to speak with power.