“The spittle of David ran down upon his beard” (1 Sam 21:13). St Augustine comments:-

Spittle signifies folly, spittle signifies weakness. But if the folly of God is wiser than men, and the weakness of God is stronger than men, let not the spittle as it were offend thee, but observe that it runs over the beard. For as by the spittle weakness, so by the beard strength is signified. He covered then His strength by the body of His weakness, and that which was exteriorly weak appeared as it were by the spittle; but within His divine strength was covered as a beard (in Ps. 33).

It is interesting that in the psalter, as in the Blessed Trinity, there is one from one (52 from 13) and one from two (107 from 56 and 59).