Sebastian Vael never tries to convert anyone!
Not once in the course of Dragon Age II does Sebastian ever try to convert another party member to the Chantry. I don't understand where this misinformation even began.
The only people he invites to the Chantry are people who are already Andrastian, in some form or another. Anders and Fenris are both already Andrastian, as declared in other parts of the game; they just don't actively participate in the Chantry the way Sebastian does. When Sebastian asks Anders if he wants to confess something, or if Fenris wants to come with him to sing or light candles, he is just trying to be open and helpful. Those are things that bring him comfort, so when he sees these other characters are troubled, he wants to help. It's really not any deeper than that, I don't think.
Half the time, Sebastian is not even the one who initiates discussion on the Chantry. He just politely responds with his invited perspective or input. One of the few times he brings it up first is with Merrill, and he is perfectly reasonable with her follow-up questions and critique. Their banter is mostly just discussing the differences between her faith and his, without any antagonism whatsoever.
Sebastian is also respectful of Tallis being Qunari in Mark of the Assassin. When other characters assume he hates her on principle, he's just like "no? why would I?" and expresses that he's happy for her, if the Qun is what makes her happy.
No, Sebastian's writing is not spotless; far from it - he was written by Hepler, after all, so what else do you expect? But it does bother me when people treat him so different from other characters who are very strong in faith, like Leliana. They are extremely similar when it specifically comes to their beliefs and the way they talk about their beliefs. And yet, to this day Sebastian gets fandom discourse that has never even touched Leliana.