One of the ways I adopted in my Introduction to Late Modern English (Edinburgh University Press, 2009) to see to what extent people had left their mark on English vocabulary was by looking for their first usages in the OED. The most prolific writer in this respect was the novelist and printer Samuel Richardson, with 245 words. With only 8 words, Lowth came disappointingly low on my list.
The OED has been under revision for a while, so I decided to check how much of these words were still attributed to him. Only six, it turned out: atrociously, bacchiac, commentatorship, distichal, intolerance and suffix (interestingly, Word only approves of three of these). Small though this set is, it can be neatly divided into two categories: technical words and words from his polemical correspondence with William Warburton. The following quotation illustrates this best:
1765 R. Lowth Let. to Warburton 62 You, my Lord, is it You of all men living, that stand forth to accuse another of Intolerance of Opinions!
With only six words left to his name, Lowth was no linguistic innovator. But would he have coined the word intolerance? A Google n-gram search suggests not. Perhaps even Warburton had used it before him. In any case, Lowth was merely an early adopter of this word, but it was to spread very quickly after him.