Albert Barnes Commentary Psalms 103:3

"Who forgiveth all thine iniquities; Who healeth all thy diseases;" — Psalms 103:3 (ASV)

Who forgiveth all thine iniquities—Pardoning all your sins. That is, it is a characteristic of God to pardon sin, and I have evidence that he has done it in my own case; this is a reason for praise. It is noteworthy that this is the first thing in the psalmist's view—the first of the "benefits" which he had received from God, or the most important among his acts or his dealings, which called for praise.

Properly considered, this is the first thing that calls for praise. That God is a merciful God—that he has declared his willingness to pardon sin, that he has devised and revealed a way by which this can be done, and that he has actually done it in our own case—is the most important matter for which we should praise him. When we understand all the things that most affect our welfare and enter most deeply into our happiness here and hereafter, we will find that this is a blessing compared with which all other favors are relatively insignificant.

Who healeth all thy diseases—Perhaps, in the psalmist's case, this refers to a particular instance in which he had recovered from a dangerous sickness. The word translated "diseases"—תחלואים tachălû'iym—occurs only in the plural form. It is translated "sicknesses" in Deuteronomy 29:22; "diseases," as here, in 2 Chronicles 21:19; "those who are sick" in Jeremiah 14:18; and "grievous deaths" in Jeremiah 16:4. It does not occur elsewhere.

This term is applicable to all forms of sickness; or, in this place, it may refer to particular diseases with which David had been afflicted. We find several allusions in the Psalms to times when their authors were afflicted with sickness; this is also true of the Psalms of David. Compare Psalms 6:2; Psalms 38:7; Psalms 41:8. The thought here is that it is a proper reason for praise to God that he has the power to heal disease.

All instances of restoration to health illustrate this, for whatever the skill of physicians or the wise application of treatments may be, healing power comes from God alone.