The Apostle...states that covetousness is the root of all sins (1 Tim 6:10). For in that passage he clearly speaks against those who, because they "will become rich, fall into temptation, and into the snare of the devil . . . for covetousness is the root of all evils." Hence it is evident that he is speaking of covetousness as denoting the inordinate desire for riches. Accordingly, we must say that covetousness, as denoting a special sin, is called the root of all sins, in likeness to the root of a tree, in furnishing sustenance to the whole tree. For we see that by riches man acquires the means of committing any sin whatever, and of sating his desire for any sin whatever, since money helps man to obtain all manner of temporal goods, according to Ecclesiastes 10:19: "All things obey money": so that in this desire for riches is the root of all sins.
--St. Thomas Aquinas, Summa Theologiae, I-II, Q. 84, Art. 1.