The priests of the machine are a secretive, reclusive sect that has a grand Quest for Knowledge, where their aim is to know the mind of the Machine God, the sum of all knowledge, by finding all information in the galaxy. The Standard Template Construct (STC) database is an important step on that road to total knowledge, and some believe it contains all human knowledge (which is probably false).

They have sixteen Universal Lores that form the core of their belief, but not all tech-priests are orthodox:

The Mysteries:

1. Life is directed motion.
2. The spirit is the spark of life.
3. Sentience is the ability to learn the value of knowledge.
4. Intellect is the understanding of knowledge.
5. Sentience is the basest form of Intellect.
6. Understanding is the True Path to Comprehension.
7. Comprehension is the key to all things.
8. The Omnissiah knows all, comprehends all.

The Warnings:

1. The alien mechanism is a perversion of the true path.
2. The soul is the conscience of sentience.
3. A soul can be bestowed only by the Omnissiah.
4. The Soulless sentience is the enemy of all.
5. The knowledge of the ancients stands beyond question.
6. The machine spirit guards the knowledge of the ancients.
7. Flesh is fallible, but ritual honours the machine spirit.
8. To break with ritual is to break with faith.

The Ninth lore is the subject of controversy - whether xenos technology counts as technology to be researched (or used). Their machine spirits are corrupted, enslaved, by the xenos, and are presumably deemed beyond saving.

Now, originally, the Omnissiah was the earthly counterpart to the Machine God (their messiah), but in time that distinction has faded (much like the Jesus/God divide has faded, since many Christians view them as one and the same). The Omnissiah was once believed to reside below the sands of Mars, but most tech-priests acknowledge the Emperor as the Omnissiah.

Tech-priests can, like all humans, vary in their orthodoxy and beliefs. The very orthodox don't touch anything non-STC with a barge pole. They often undergo the unction of pure thought, and have the creative side of their brain lobotomised, becoming almost robot-like. Likewise, the moderate tech-priest may dabble with studying xenos tech, but doesn't dare use it, and radicals might use xenos tech wholesale. Likewise the study of the warp.

This is a bit stereotypical, admittedly. You get orthodox tech-priests who are still very emotional, and radicals with the unction of pure thought and are very dogmatic in their beliefs.

Tech-priests don't go around revealing their knowledge to anyone. What a group of tech-priests discovers is for their ears and those of their superiors only (and sometimes not their superiors or even each other, if they're particularly secretive). But the bulk of them work in teams and acknowledge the advantage of knowledge dissemination amongst their peers, whilst keeping the competitive edge over their rival teams.