Newton's estimate of the age of the Earth
The estimates of Newton and Kepler, and the similar estimates of dozens of their contemporaries, were produced by treating the Bible as a historically accurate document and deriving a chronology from it. I think that a detailed explanation of their techniques would be off-topic for Earth Science, but the Wikipedia page on the Ussher Chronology (which provides probably the most famous biblically-based estimate of the Earth's age) goes into reasonable detail about the methods used for such calculations.