Scientists have faith in their theories when they have good data/evidence to support those theories. The faith that scientists have in the their theories is always provisional, that is, if new data/evidence overturns their theory they lose their faith in it PDQ. Religious people often have faith in their beliefs when there’s no data/evidence to support those beliefs or even when the data/evidence goes against their beliefs. Religious people see their kind of faith as a virtue. When applied to scientific investigations, scientists think religious-type faith is a detriment. Sometimes religious people willfully conflate those two different meanings of faith in an attempt to show that science is just like a religion.
First of all, the majority of the things that scientists have faith on may have certain evidence, the models may be very good, but it is more of a hope based on probability. It is honestly a house built on sand. We have seen how it took nothing for such valid systems as the Ptolemaic (geocentric) model and concepts such as the luminiferous aether to crumble to dust before a new discovery. Today we often talk of dark matter and dark energy in a similar way, attempting to hold together the existing models.
Faith in the science is, in many aspects, a hope that the model will work, that it will be the best approximation to reality.
Religious faith, indeed, is different. While we often confuse faith with superstition, and we call many things “faith”, true faith is built on the rock, and the rock is God. While some may not believe in His existence, those who do actually know that He exists, not just believe in His existence. Evidence does not overturn their beliefs, and there is actually plenty of evidence in support of their beliefs. That is why ever since man exists religion has existed, and that is why even in the modern age there are billions who believe in God, and the majority follow the monotheistic view of one God, the most illuminated one.
The best writing I have ever read on this topic is called
Faith and Reason.
However, we have writings from very ancient times that witness how the faith of scientist and the faith of religious were, in fact, based on hope, the same hope, and what now many scientists find amusing - the existence of God - was once (and still is for many other wise scientists) an obvious fact.
The laws of nature are nothing but the mathematical thoughts of God. Euclides, 300 b.C.
The material world and the spiritual world have the same origin in the Creator. The values of science cannot in any way substitute those of revealed truth. If we were truly living in the age of science, then everyone would grasp this truth. Galilei, 1600
This most beautiful system [The Universe] could only proceed from the dominion of an intelligent and powerful Being.” Isaac Newton, 1642
A little bit of science drives away from God, but a lot brings back to Him. Louis Pasteur, 1822
One year spent in the study of artificial intelligence is enough to make us believe in God. Alan Perlis, 1950