Lead me in thy truth - In the way which thou regardest as truth, or which thou seest to be true. Truth is eternal and unchanging. What God sees and regards as truth is true, because he sees things as they are; and when we have the divine estimate of anything, we understand what the thing is. It is not that he makes it to be true, but that he sees it to be true. Such is the perfection of His nature that we have the utmost assurance that what God regards as truth is truth; what He proclaims to be right is right. It is then His truth, as He adopts it for the rule of His own conduct, and makes it known to His creatures to guide them.
And teach me - Since this would be understood by the psalmist, it would be a prayer that God would teach him by His law as then made known; by His Spirit in the heart; by the dispensations of His providence. As applicable to us, it is a prayer that He would instruct us by all the truths then made known, and all that have since been revealed; by His Spirit in its influences on our hearts; by the events which are occurring around us; by the “accumulated” truth of ages; the knowledge which by all the methods He employs He has imparted to people for their guidance and direction.
For thou art the God of my salvation - The word “salvation” is not to be understood here in the sense in which it is now commonly used, as denoting deliverance from sin and future ruin, but in the more general sense of “deliverance” - deliverance from danger and death. The phrase is synonymous with “preservation,” and the idea is that the psalmist regarded God as his preserver; or that he owed his protection and safety in the time of danger to Him alone.
On thee do I wait - That is, I rely on Thee; or, I am dependent on Thee. He had no other source of reliance or dependence.
All the day - Continually, always. He was really dependent upon Him at all times, and he felt that dependence. It is always true that we are dependent upon God for everything; it is not true that we always feel this. It was a characteristic of the piety of the psalmist that he did feel this.