The is the definite article: and relates to sin and folly; according to Rule 1st, which says, "Articles relate to the nouns which they limit." Because the meaning is—the sin or folly.
Sin is a common noun, of the third person, singular number, neuter gender, and objective case: and is governed by from; according to Rule 7th, which says, "A noun or a pronoun made the object of a preposition, is governed by it in the objective case." Because the meaning is—from sin.
Or is a disjunctive conjunction: and connects sin and folly; according to Rule 22d, which says, "Conjunctions connect words, sentences, or parts of sentences." Because the meaning is—sin or folly.
Folly is a common noun, of the third person, singular number, neuter gender, and objective case; and is connected by or to sin, and governed by the same preposition from; according to Rule 7th, which says, "A noun or a pronoun made the object of a preposition, is governed by it in the objective case." Because the meaning is—from sin or folly.
In is a preposition: and shows the relation between indulge and which; according to Rule 23d, which says, "Prepositions show the relations of words, and of the things or thoughts expressed by them." Because the meaning is—indulge in which—or, which they indulge in.
Which is a relative pronoun, representing sin or folly, in the third person, singular number, and neuter gender; according to Rule 13th, which says, "When a pronoun has two or more antecedents connected by or or nor, it must agree with them singly, and not as if taken together:" and is in the objective case, being governed by in; according to Rule 7th, which says, "A noun or a pronoun made the object of a preposition, is governed by it in the objective case." Because the meaning is—in which;—i. e., in which sin or folly.
The is the definite article: and relates to multitude; according to Rule 1st, which says, "Articles relate to the nouns which they limit." Because the meaning is—the multitude.
Multitude is a common noun, collective, of the third person, conveying the idea of plurality, masculine gender, and nominative case: and is the subject of indulge; according to Rule 2d, which says, "A noun or a pronoun which is the subject of a finite verb, must be in the nominative case." Because the meaning is—multitude indulge.
Thoughtlessly is an adverb of manner: and relates to indulge; according to Rule 21st, which says, "Adverbs relate to verbs, participles, adjectives, or other adverbs." Because the meaning is—thoughtlessly indulge.
Indulge is a regular active-transitive verb, from indulge, indulged, indulging, indulged; found in the indicative mood, present tense, third person, and plural number: and agrees with its nominative multitude; according to Rule 15th, which says, "When the nominative is a collective noun conveying the idea of plurality, the verb must agree with it in the plural number." Because the meaning is—multitude indulge.