Q: could not understand the psychology behind it. Term used Radical follower in matters of faith and not in other matters ?
Can you define a citizen “ radical one” who believes and follows law of a country in letter and in spirit?
Can a law enforcer be called a “radical” policeman if he/ she enforces law to its letter and sprit?

A:

This is a sharp observation, and you’re right that there’s an asymmetry worth pulling apart — it’s not really about psychology of individual people so much as how the word itself gets used differently depending on domain.
Q: Why isn’t a strict law-abiding citizen called “radical”?
Because in secular/civic contexts, “radical” has come to mean something closer to extreme relative to the political center or advocating fundamental upheaval of the existing system — not “thorough in compliance.” A citizen who pays every tax, follows every traffic law, and votes in every election isn’t called radical; they’re called a “model citizen” or “law-abiding.” The word “radical” gets reserved for people who want to change the system root-and-branch (the Latin radix = root) — radical reformers, radical revolutionaries — not for people who follow the existing system precisely.
Q: So why does “radical” get attached to religious observance specifically?
A few distinct things are getting bundled under one word, and that’s likely the source of your puzzlement:
1. Media and political usage shifted the word’s meaning in religious contexts specifically. Since the late 20th century, “radical” became closely associated with “radicalization” — a specific term in security/counterterrorism discourse describing the process toward violence, not mere strict observance. Once that association formed, “radical Muslim,” “radical Christian,” etc. carry a connotation of political violence or extremism, even when used loosely to mean “very devout.”
2. Conflation of strictness with extremism. A person who prays five times a day, fasts, dresses modestly, and avoids alcohol is performing personal religious diligence — analogous to the law-abiding citizen. But because “radical” in the religious context has been pre-loaded with the security/violence connotation, strict observance gets misread as a step toward extremism, when structurally it’s the same thing as the citizen who follows every law in letter and spirit.
3. Visibility and “otherness” bias. Religious practice is often visibly different from majority culture (dress, dietary restriction, prayer times) in ways that civic compliance isn’t. Visible difference draws more scrutiny and a readier label, even absent any behavioral comparison to extremism.
Q: Can a police officer who enforces the law to its letter and spirit be called “radical”?
Generally, no — and this confirms the asymmetry you’re noticing. Such an officer would be called “by-the-book,” “strict,” or “a stickler for the rules” — possibly even criticized as inflexible — but “radical” wouldn’t be the natural word, because there’s no equivalent “radicalization” framework attached to law enforcement discourse. If anything, an officer who deviates from procedure (excessive force, ignoring due process) is the one more likely to be called “extreme” in commentary — interesting that the polarity flips here: deviation, not compliance, draws the charged language.
Q: Is there a fair, non-loaded term for strict-but-non-violent religious observance?
Terms like “devout,” “observant,” “practicing,” or “orthodox” (lowercase) serve that function without the security-discourse baggage “radical” now carries. Scholarship on religion and politics (e.g., work distinguishing fundamentalism as theological literalism from extremism as willingness to use violence to impose belief) tries to maintain exactly this distinction — though in casual and media usage, the words get flattened together, which is presumably the root of the unease you’re describing.
So your instinct is correct: there’s no principled reason “radical” should describe rigorous faith-compliance but not rigorous law-compliance or rigorous law-enforcement. The asymmetry is a function of how the vocabulary developed in security and media discourse over the last few decades, not a logical or psychological truth about the people being described.