67 It must have been on account of this inspection of the depths of David's heart by the Spirit of God that, when on being reproved by the prophet, he said, I have sinned, he was considered worthy to be told, immediately after this brief confession, that he was pardoned— that is, that he was admitted to eternal salvation. For he did not escape the correction of the fatherly rod, of which God spoke in His threatening, that, while by his confession he obtained eternal exemption, he might be tried by temporal chastisement.
And it is a remarkable evidence of the strength of David's faith, and of his meek and submissive spirit, that, when he had been told by the prophet that God had forgiven him, although the threatened consequences were still permitted to follow, he did not accuse the prophet of having deluded him, or murmur against God as having mocked him with a declaration of forgiveness. This deeply holy man, whose soul was lifted up unto God, and not against God, knew that had not the Lord mercifully accepted his confession and repentance, his sins would have deserved eternal punishment.
So when, instead of this, he was made to smart under temporal correction, he saw that, while the pardon remained good, wholesome discipline was also provided. Saul, too, when he was reproved by Samuel, said, I have sinned. Why, then, was he not considered fit to be told, as David was, that the Lord had pardoned his sin? Is there acceptance of persons with God? Far from it. While to the human ear the words were the same, the divine eye saw a difference in the heart. The lesson for us to learn from these things is, that the kingdom of heaven is within us, and that we must worship God from our inmost feelings, that out of the abundance of the heart the mouth may speak, instead of honoring Him with our lips, like the people of old, while our hearts are far from Him.
We may learn also to judge of men, whose hearts we cannot see, only as God judges, who sees what we cannot, and who cannot be biased or misled. Having, on the high authority of sacred Scripture, the plainest announcement of God's opinion of David, we may regard as absurd or deplorable the rashness of men who hold a different opinion. The authority of Scripture, as regards the character of these men of ancient times, is supported by the evidence from the prophecies which they contain, and which are now receiving their fulfillment.
Source: Reply to Faustus the Manichaean (New Advent)