The word “Christian” can mean a lot of things to different people, like “a good person” or “someone from Western culture”. We will take an “active Christian” to be a person who believes the Bible to be inspired and practices their Christian faith in an active way.
Contrary to what atheists often imply, many great scientists in history were not irreligious or even just nominally religious but had an active Christian faith. On the other hand, practically all of them believed that science should be done by observation, and independently of biblical interpretation. In this latter issue they differed from the modern “young earth creationists” who list them as “believing the Bible” on web sites like http://creationsafaris.com/wgcs_toc.htm. These great scientists were not ‘biblical literalists’ in the modern sense. They were generally sincere, bible-believing Christians but they neither took their bible “literally” eg on the creation accounts, nor derived their science from it.
Some of the earliest stirrings of science came in the 13th century Oxford Franciscan monks (like Occam) and the continental Catholic scholastics (like Aquinas). The Franciscans were not “nominal Christians” but active in faith and often in a Christ-like concern for the poor. Active Christians have then been at the forefront of virtually all the different sciences.
Francis Bacon (1561-1626) became hugely influential regarding the scientific method, and Puritans like John Wilkins (1614-1672), who developed his ideas, were largely behind the Royal Society. Key early members of this included Robert Boyle(1627-1691) who wrote a key book The Skeptical Chemist which demolished alchemy, discovered the gas laws, co-discovered phosphorous, invented the match, measured the density of air, first to distinguish acids/bases/neutral substances, introduced the litmus test, etc. He also learned Greek and Hebrew to read the bible better because his faith was so important to him.
In biology, Boyles associate the devout Christian John Ray (1627-1705) did the foundational work in species classification, that was developed into our modern system by the Christian Carl Linnaeus(1707-1778) Professor of Medicine and also Botany at Uppsala (Sweden). Devout Anglican Richard Owen (1804-1892) coined the word 'dinosaur', established the Natural History Museum, and was the greatest comparative anatomist of the age. The work in biology continued eg with the evangelical Harvard botanist Asa Gray (1810-1888), who was Darwin’s first American confident and supporter. Ironically, two of the key Darwinists the atheist Richard Dawkins identifies in the twentieth century were David Lack (1910-1973) who became a Christian in his late 20’s, and R A Fisher (1890-1962) who regularly preached in his Cambridge college chapel. In genetics, of course, whilst Darwin’s sceptical cousin Francis Galton was pronouncing that the “priestly mind was not conducive to science” the breakthrough in his chosen area of genetics was being made by an Austrian Monk Gregor Mendel (1822-1884).
In medicine, devout Christians Louis Pasteur (1822-1895) was a renowned chemist, bacteriologist, invented vaccination, pasteurization, sterilization & immunization, whilst Joseph Lister (1827-1912) developed antiseptic surgery.
In astronomy, the Franciscan Roger Bacon (who developed scientific method) developed the model of Ptolemy with a static but small and spherical earth in a large universe. Tycho Brahe (1546-1601) was a devout Lutheran who pioneered really accurate astronomical observation, recorded the first new star in detail and made the first complete study of a comet. His younger associate, Johann Kepler (1571-1630), one of the very greatest scientists ever, was also a devout Lutheran. Kepler pioneered in optics , discovered the laws of planetary motion, invented the idea of gravity as action at a distance, contributed to the development of calculus – and invented science fiction on an imagined trip to the moon. The young British genius Jermiah Horrocks, who in 1649 first observed the transit of Venus, was a devout Puritan. Isaac Newton himself (1642-1726) was devoutly religious if unorthodox. William Herschel (1738-1822) who discovered the planet Uranus, began infer red astronomy, and started astrophysics was a believer, as was his astronomer son John Herschel (1792-1871). Astronomer Royals like George Biddell Airy, John Couch Adams, and leading figures like the Earl of Rosse were devout. William Huggins (1824-1910) who, with his wife, developed astrophysics was a Christian, and Professor E H Maunder (1851-1928) the president of the British Astronomical Association (who continued the work into the twentieth century) was one of the few early Pentecostal Christians. The Big Bang idea came from a Belgian astronomer priest Georges Lemaitre (1894-1966). Even more recently, pulsars were found in 1967 by Quaker Jocelyn Bell and Nobel prizewinner Anthony Hewish, an active Anglican Christian who appears on this web site!.
In maths and physics, Rene Descartes (1597-1650) (a devout Catholic who wanted his ideas to be adopted by the Jesuits) developed ideas of light, invented linera inertia, and made fundamental contributions to mathematics. Blaise Pascal (1623-1662) was a mathematician, hydrostatics and hydrodynamics pioneer, laid the foundations for conic sections, differential calculus & probability theory, invented the barometer. Christian Huygens (1629-1695) was an astronomer, mathematician, and physicist, who introduced the pendulum clock, etc. Newton we already mentioned. In the nineteenth century the prominent physicists were: James Joule (1818-1889) George Stokes (1819-1903), Lord Kelvin (William Thomson) (1824-1907), David Brewster (1781-1868), James Clerk Maxwell (1831-1879) and Lord Rayleigh (1842-1919) – all were unusually devout Christians. Michael Faraday (1791-1867), was one of the great physicist/chemists of all time who developed electricity etc, and someone to whom his Christian faith meant everything.
In geology, the development of stratigraphical ideas was by the monk Nicholas Steno (1631-1686). Jean Deluc (1727-1817) coined the word 'geology'. The real birth of modern geology in the nineteenth century owed much to the devout Protestant Georges Cuvier (1769-1832). Other figures were Rev William Buckland (1784-1856) at Oxford and at Cambridge the devout evangelical geologist Adam Sedgwick (1785-1873). Sedgwick mapped more of the geological strata than any other person, and modern geology has accepted his ideas of periodic geological catastrophes (rather than the denial of them by Charles Lyell - himself a nominal Christian vetted for Christian orthodoxy when appointed by University College London in 1831). Prominent American geologists included active Christians Benjamin Silliman (1779-1864) president of the Association of American Geologists etc, Edward Hitchcock (1793-1864), and James Dana (1813-1895) who succeeded Silliman at Yale, edited the American Journal of Science, was president of the Geological Society of America, president of the American Association for the Advancement of Science, etc.
All these were sincere active Christians in times when many in their surrounding society were irreligious or disinterested. Often they were interested in understanding the physical world exactly because they believed it the creation of a personal and loving creator. Of course the occasional scientist has been agnostic or irreligious, but the general trend has always been otherwise. Biology, geology, physics, astrophysics, medicine, and even aspects of evolutionary biology, have all been pioneered and developed by active bible-believing Christians. Why should we believe that the world behaves rationally? Because it is made by a rational God. Why should we believe that our minds are capable of understanding truth about it? Because we are made in the image of that God. If, as atheists usually believe, we and our minds are the product of a blind undersigned process gauged only to help us survive, then why should we be able to arrive at truth at all?