Is participation in religious and social forums permissible, whether by posting topics, replying to them, or expressing opinions therein?

May Allah reward you for your concern in helping others. If you are qualified scientifically and religiously, there is no objection to you taking the lead in answering people's religious questions; rather, it is a great good. The Prophet, peace and blessings be upon him, said: "By Allah, for Allah to guide one man through you is better for you than red camels," and he also said: "The one who points to good is like the one who does it." However, if you are not qualified, then it is not permissible for you to do so, because it is speaking about Allah without knowledge, and that is a grave sin. Allah the Almighty said: "And do not say about what your tongues falsely describe, 'This is lawful and this is unlawful,' to invent falsehood about Allah. Indeed, those who invent falsehood about Allah will not succeed. A brief enjoyment, and for them is a painful punishment," and He also said: "And do not pursue that of which you have no knowledge."