During this period he became interested in evolution and constructed a workshop, which he called his laboratory, which he used to take pictures of microscopic organisms and other objects to illustrate his lectures and books, though rarely for actual experiments. Rimmer enrolled in a correspondence course in geology (at the University of Colorado, while on a speaking tour in the Rocky Mountains region) and started collecting fossils. Thereafter his lectures increasingly featured evolution and he acquired a reputation among fundamentalists as a qualified critic of evolution, with a fundamentalist journalist declaring:
Dr. Harry Rimmer is one of the most remarkable young men in the country. He is the rare combination of a scientist, who is a successful soul-winning evangelistBy the time he had achieved this prominence Rimmer had founded, on paper, the Research Science Bureau, which "existed primarily, if not exclusively, to underwrite Rimmer's ministry and occasional field trips." In return members received copies of Rimmer's pamphlets and, for a short period in 1927, a monthly newsletter. However with the Great Depression, funding for the Bureau dried up, and it went into abeyance until the early 1940s, with Rimmer's creationist efforts instead being channelled through William Bell Riley's World Christian Fundamentals Association. In 1934, Rimmer became pastor of the First Presbyterian Church of Duluth, Minnesota, on the understanding that he would spend six months of the year writing and lecturing.
A writer for the Debunker, who attended one of Rimmer's lectures, described that his "scientific" method as:
to accept the Bible as absolute truth and discard, ignore, or hopelessly distort all facts that had been gathered, until the remaining facts in their distorted nature 'proved' the religious conclusions which had been established even before the facts had been consulted.
He was awarded an honorary DSc by the Wheaton College. He claimed to have visited 4000 high school assemblies in a 25 year period, or about one every two days. His principal theme was that "There are no scientific errors in the Bible."
He was the author of several books including Dead Men Tell Tales, Harmony of Science and Scripture, and Modern Science and the Genesis Record. His books sold well; some in the hundreds of thousands. However his books came under considerable scientific scrutiny and criticism. When the Christian American Scientific Affiliation's publisher, Van Kampen Press, contemplated republishing them, the ASA performed an evaluation of a representative sample which was highly critical, with even recommending against publication. Edwin Y. Monsma (who would later become one of the co-founders of the Young Earth creationist Creation Research Society) gave the opinion that Rimmer's The Theory of Evolution and the Facts of Science should not have been published in the first place, contained "inaccuracies and overstatements" and relied upon ridicule.
Rimmer simply loved to debate; he would debate anyone--atheists, religious scientists, college faculty, even fellow fundamentalists. In a famous instance, he debated another creationist, William Bell Riley about the nature of the days in Genesis. He was apparently a colorful speaker and some called him the "noisiest evangelist in America".
He wrote that, "In all of our scientific progress we have not yet discovered one single fact that contradicts or refutes any statement in the Bible." He had a gift for producing unlikely explanations to protect the veracity of the biblical text. For example, Rimmer stated that Jonah could live after being swallowed by a whale because Rimmer postulated that there is special cavity in the heads of whales which are the whales' "breathing tanks" for underwater breathing. Rimmer also insisted that the passage where rabbits chew their cuds is the result of a mistaken translation, and claimed that camels do not have cloven hooves.
Rimmer tried to use science to prove the veracity of the Bible. One of the sources he relied on was a speculative book by Joshua's Long Day and the Dial of Ahaz published by Charles Totten, an instructor in Military Science at Yale. In 1890. Rimmer claimed that the Bible story in which Joshua ordered the sun to stand still in the heavens had been definitely proved by a Yale Professor (Totten). A updated version of this claim has it that some NASA scientists discovered a missing period of 24 hours. He also promoted a version of the urban legend of James Bartley and a sperm whale, which he used to uphold a literal interpretation of Jonah.
On two occasions claimants took Rimmer to court. The first was in 1929 when a retired army colonel challenged the story of God feeding the children of Israel in the wilderness by sending so many quail that they were piled two cubits high for a days journey around the camp, which he calculated would require over 29x1012 quails (or over 12 million per Israelite). However the judge, apparently reasoning that Moses was a more reliable witness than the colonel, ruled against him. In 1939, the Research Science Bureau upped the offer to $1000.
Floyd disclaimed interest in the money, describing his intent as "to convince fundamentalists, through court judgement[,] that there are such errors in the Bible."
During the trial, Rimmer defended the Bible with statements such as "You could get two of every species of insect on the hides of two good-sized elephants, and they would not, therefore, occupy any additional space in the ark" and "most all present-day scientists have completely discredited the theory of the record of the rocks." Rimmer said of his testimony, "I showed how God pushed the clouds further back and made what the aviators call a 'ceiling' between the clouds and the substance of the earth, and God called it a 'firmament' and men call this cloudy ceiling 'heaven'.
Rimmer won the case on a technicality: that the particular newspaper advertisement that Floyd responded to was not placed by Rimmer himself. Despite this, Rimmer claimed that the trial "ended in legally establishing the position of all who hold that the Word of God is inerrant".
The trial in a New York court is described by his defense attorney James E. Bennet in his book The Bible Defeats Atheism - A Story of the Famous Harry Rimmer Trial as told by Attorney for Defendant James E. Bennet.