That's what I intended to do as well, but it made no narrative sense. The point of going to Kvatch was to fetch said person to safey, and going dungeon crawling to open the gates of the castle while he's in the church (or in toll) just felt wrong, especially since none of the guards there even acknowledged his presence.
Now that I think about it, I'm pretty sure that you're supposed to not talk to him until you're done with the castle quest. But damn, you'd think that after hyping up the narrative around the guy they'd expect you to search for him first as opposed to trying to retake the city, especially since he was standing right there in the chapel.
<{POST_SNAPBACK}>
The linear nature of the main plot really hurt the game in that sequence. Arriving at Kvatch, there's a big freaking gate, some survivors have gathered at the church, one of whom might be the one you seek. Bam, instant opportunity to make significant decisions. Do I risk the portal, helping out the city in the long run, but potentially delaying the retrieval of my primary target? Do I drop everything else and just focus on ensuring my target's safety?
Oh. The gate blocks the only way into the city! I have to turn it off before I can do anything else! Joy. Rollercoaster ride