How do I avoid my players finding their end goal too early?

Howdy reddit, I'm in a bit of a pickle and I could use some help! Sorry if my thoughts are a bit muddled. Obligatory warning to Alison, Iyah, and Francois-- if you're reading this, stop now! Beau the cat's life hangs in the balance!

So, I've been running a 5e game twice a month since last March, and it's been going great! Really, these are the players I've always wanted, and the game couldn't be going better. It's an open world style game during a sort of biblical apocalypse, inspired by a lot of Fromsoftware games. We have a monk/rogue, a paladin, and a sorcerer/wizard, all at lvl 3. The way I've structured the game is that they receive smuggling "quests" from the mob bosses they work for, but I give them ample time to explore the world, their home towns, and their personal stories. My players just got their new mission- head to a dying city and give a skeleton key to a captive noblewoman! This city is also pretty lore relevant to the wizard! When I gave them the mission guidelines, I specifically gave them extra travel time so that they could explore some of the other nearby areas, hoping that the monk would want to explore a nearby city that's lore relevant to his character. It all worked well, they seem excited to go to the city, and the monk is hoping to stop at the adjacent city, too!

Okay, it maybe worked TOO well... in a way I really should have foreseen. So, the sorcerer/wizard worships a god who went missing 10 years ago, the god of fortune, divination, and fate. He started as a wild magic sorcerer after his town was literally swallowed by a "time rift" and he was spat back out. This god also has a testing ground, a ruin used as a pilgrimage by the god's followers, important to becoming a priest... which is where the god has been locked away by an enemy church. I've hinted to the importance of this ruin many times, and I should have seen this coming, but it's nearby the city they're heading to, and he wants to swing by on the way home. Problem is, I explicitly plan for this to be the character's personal endgame. We've even discussed his vision for late game-- he wants a choice that either decides if he redeems himself for his actions and becomes a cleric of his god, or slips further down the moral pit and becomes a full wild magic sorcerer... I cannot stress enough how perfectly this aligned with the ruins and how I wanted it to go. But they're only level 3! The other 2 players have barely scratched the surface of their backstories, I can't have him go and save his god right away!

So Reddit, I come to you. I normally would roll with the punches and just put some lore relevant info in the ruin, moving the god, but I don't know where I'd place the captive god then... I'm trying to think of a way of barring his ability to immediately go into this dungeon to save his god. Maybe they need a special item to get inside, or there's some kind of ancient warrior that is the first test that won't let them by unless they're strong enough? Maybe there are some paladins of the rival church camped out front to stop anyone from entering? I want to surround the area with wild magic storms, so maybe the storms are just too chaotic and they can't pass until they're stronger? I'd really prefer not to move the location of the god, so does anyone have any idea of how I can handle this, or in general how to avoid players getting to certain end areas too soon?

Thanks!