The correct preposition is "to" because the phrase "stick to" is a standard English idiom meaning to remain loyal to, continue following, or firmly adhere to a belief, decision, rule, promise, or plan. Therefore, the sentence "Please stick to your faith." means "Please remain steadfast in your faith" or "Do not abandon your beliefs." The expression stick to is commonly used in both spoken and written English with nouns such as faith, principles, rules, schedule, plan, topic, and decision. It is an important collocation frequently tested in grammar and competitive examinations.
Prepositions often form fixed combinations with verbs, and these combinations must be memorized because changing the preposition usually changes the meaning or makes the sentence grammatically incorrect. For example, we say stick to the rules, stick to your promise, stick to the plan, and stick to the truth. Understanding such verb-preposition combinations improves both written and spoken English and helps candidates answer grammar-based multiple-choice questions accurately.
Why the other options are incorrect:
- On is incorrect because stick on generally means to attach something to a surface, not to remain faithful to a belief.
- With is incorrect because stick with can mean to stay with a person or continue using something, but the standard expression for beliefs or principles is stick to.
- In is incorrect because stick in refers to inserting or becoming lodged inside something and is not appropriate in this context.