It's important to clarify that scientists don't place "faith" in evolutionary theory in the religious sense. They base their understanding of evolution on evidence, observation, and rigorous testing. However, there are core principles that underpin the theory of evolution:

1. Common Descent: This is the idea that all life on Earth shares a common ancestor. This is supported by overwhelming evidence from various fields, including genetics, paleontology, and comparative anatomy.

2. Natural Selection: This is the mechanism by which evolution happens. It states that organisms with traits better suited to their environment are more likely to survive and reproduce, passing those advantageous traits to their offspring.

3. Variation: Organisms within a population are not identical. There are variations in their traits, some of which are heritable. This variability is crucial for natural selection to act upon.

4. Gradualism: Evolution is not a sudden process but rather a slow, gradual change over long periods. This is supported by the fossil record, which shows a progression of changes in organisms over millions of years.

It's important to note that the theory of evolution is not about "belief" but about scientific understanding based on evidence and observation. While the above principles are fundamental, they are constantly being refined and tested with new discoveries.