Good and upright is the Lord - His character is benevolent, and he is worthy of confidence. He is not merely “good,” but he is equal and just in his dealings with people. This latter attribute is no less a reason for confidence in his character than the former. We need a God who is not merely benevolent and kind, but who is just and faithful; whose administration is based on principles of truth and justice, and in whose dealings, therefore, his creatures can repose unlimited confidence.
Therefore will he teach sinners - Because he is good and upright, we may approach him with the assurance that he will guide us aright. His “goodness” may be relied on as furnishing evidence that he will be “disposed” to do this; his “uprightness” as furnishing the assurance that the path in which he will lead us will be the best path. We could not rely on mere benevolence, for it might lack wisdom and firmness, or might lack power to execute its own purposes; we can rely upon it when it is connected with a character that is infinitely upright, and an arm that is infinitely mighty.
In the way - In the right way - the way in which they should go, the path of truth, of happiness, of salvation.