Definition of "zealous"

I had almoſt forgotten to acquaint your honor, that one major Alford (who was in mr. Love's conſpiracy) was of the graund inqueſt at Saliſbury, and was very zealous in his highneſſe ſervice here, and his good affection and wiſe carriage here, did much advantage the buſſineſe.

1655 April 21, “Mr. Ja. Nutley to ſecreary Thurloe.”, in A Collection of the State Papers of John Thurloe, Eſq; […], volume III, London: […] the Executor of the late Mr. Fletcher Gyles; Thomas Woodward, […] Charles Davis, […], published 1742, page 399

Johnson was truly zealous for the success of "The Adventurer;" and very soon after his engaging in it, he wrote the following letter:

1791, James Boswell, “(please specify the year)”, in The Life of Samuel Johnson, LL.D. […], London: […] Henry Baldwin, for Charles Dilly, […]

Doubtless many will exclaim against the Roman Catholic Church for this; but the simple truth is that Protestantism was no less zealous against the new scientific doctrine.

1896, Andrew Dickson White, A History of the Warfare of Science with Theology in Christendom, published 2004, page 122

[…] and there were few more zealous dancers at the fashionable balls in the Raleigh Tavern at Williamsburg.

1940, Foster Rhea Dulles, America Learns to Play: A history of popular recreation, 1607-1940, page 61

Newt Gingrich […] left Congress in 1998, following GOP midterm-election losses that many blamed on his zealous pursuit of Bill Clinton's impeachment.

2011 April 4, “Newt Gingrich”, in Time, retrieved 9 Sept 2013