What is the ruling on debating an ordinary person who has deviated from the religion or a sinner (one who follows his whims) in front of a gathering of people, and is it obligatory to refute a specious argument if it is presented in a gathering of people?

There are numerous narrations from the early generations (Salaf) warning against sitting with people of innovation (bid’ah) and desires (ahwa), fearing that some of their doubts might cling to the heart, leading astray those who associate with them. As for one who possesses knowledge to refute their doubts, it is appropriate for him to debate them, and it may even be obligatory. However, for one who lacks knowledge, it is not permissible for him to delve into matters of the Shari'ah, due to the Almighty’s saying: (And do not pursue that of which you have no knowledge. Indeed, the hearing, the sight, and the heart - all of those will be questioned.)