Drearyweather,
Psalm 51:5 is known to any average Bible reader. But this particular thought is not shared by other OT writers, hence it is like an isolated thought. In OT we find many references to sinless persons (Gen 5:24; Ezekiel 14:14 and compare Habakkuk 2:4) Interestingly, even God does not think humans are sinful from birth. (Job 1:8) What was an isolated thought was launched as main subject by Paul, thus Paul became the Father of this school of thought that says we are all sinners by birth. Because people like the idea that we are sinners by birth because of which we are not responsible for the sins we commit, nobody bothered to think that this is actually Paul’s experience not of mine, and experiment to the contrary.