What is the way to guide someone who advocates secularism and publishes indecent pictures on social media, and what are the arguments and proofs to convince them?

One must beware of poisoned pages, and only those who have been granted a share of knowledge should confront and debate the specious arguments of the misguided, for hearts are weak and doubts are insidious. Whoever is not qualified for that should avoid looking into their falsehoods and beware of temptation. As for the qualified, let them explain to them with wisdom and good counsel that the matter belongs to Allah alone, and it is not for His servants to rule by other than His Sharia, and that Allah's Sharia contains benefits for His servants, and that whoever deems it permissible to seek judgment by other than Allah's Sharia has exited the fold of Islam.