Bsrgvty

If we turn something (including a concept or tactic) into a weapon, we have weaponized it. If we legally classify something as a crime, we have criminalized it.

What is an equivalent single-word, verb term for turning something into or classifying something as a sin?

Best possible answers will capture the religious overtones adjacent to the word 'sin,' although are not restricted to western religions. Good but not best answers will at least capture a sense of newly classifying something as immoral without the direct overtones. Merely capturing a sense of modern illegality (i.e., criminalize, above) is not enough.

Its use in a sentence might be, "It was not enough for the preacher to criminalize loud music; he felt the need to ____ it, to turn it into a mortal sin."

condemn

to criticize something or someone strongly, usually for moral reasons.

This word can be used for moral (not merely legal) denunciation of an individual, a group or their actions.

Example:

In Jewish and Christian tradition, the love of money is condemned as a sin primarily based on texts such as Ecclesiastes 5:10 and 1 Timothy 6:10. The Jewish and Christian condemnation relates to avarice and greed rather than money itself.

I would suggest

Demonise/Demonize

Although it seems to mostly get applied to people, quite a few of the examples are of other nouns:

Finally, we must not demonize any experience or initiative, whether negative or positive.

I've certainly heard it in conversation to describe something undesirable (to the point of being demonic)

In this particular context of religion, I think it works really well.

anathematize

to pronounce an anathema upon

Source: Merriam-Webster

Let's examine M-W's definition of anathema:

1a : one that is cursed by ecclesiastical authority

b : someone or something intensely disliked or loathed —usually used as a predicate nominative
//… this notion was anathema to most of his countrymen.

— Stephen Jay Gould

2a : a ban or curse solemnly pronounced by ecclesiastical authority and accompanied by excommunication

b : the denunciation of something as accursed

c : a vigorous denunciation : curse

Particularly due to the connotation of an ecclesiastical authority making an official pronouncement in 1a, I think this best captures the OP's request.

I would use the verb moralize:

[Merriam-Webster]
1 : to explain or interpret morally

// an essay moralizing about the evils of alcohol

With the example sentence:

It was not enough for the preacher to criminalize loud music; he felt the need to moralize it, to turn it into a mortal sin.

I would use "stigmatise":

From WordNet (r) 3.0 (2006) [wn]:

stigmatise
v 1: to accuse or condemn or openly or formally or brand as
disgraceful; "He denounced the government action"; "She was
stigmatized by society because she had a child out of
wedlock" [syn: stigmatize, stigmatise, brand,
denounce, mark]
2: mark with a stigma or stigmata; "They wanted to stigmatize
the adulteress" [syn: stigmatize, stigmatise]

There is a subtle difference to "condemn" in that "stigmatise" carries the connotation of vilifying (oops, another option) something that is not a priori considered a bad thing. So I can "condemn Paul's greed" but it would sound weird to "stigmatise Paul's greed". Basically, I don't see that "condemn" meets your requirement of "captur[ing] a sense of newly classifying something as immoral".

taboo

1. a social or religious custom prohibiting or restricting a particular practice or forbidding association with a particular person, place, or thing.

"many taboos have developed around physical exposure"

Source: Lexico

Your sentence would be:

It was not enough for the preacher to criminalize loud music; he felt the need to make it taboo, to turn it into a mortal sin

The church would say that society can make something into a crime or something else into a weapon but the church has already decided what is a sin and has even categorized which are mortal sins and which venal sins. I don't believe a word exists.