Early on in Jackson’s first LOTR movie, Gandalf exercises great caution in handling the ring, telling Frodo that he (Gandalf) would be vulnerable to manipulation by its evil power. So he insists that Frodo take the ring to Rivendell.
He knew that Bilbo was able to possess the ring for a long time, and it didn’t corrupt him completely. Frodo, being Bilbo’s nephew and having a similar disposition (and inheriting the more Tookish traits of the family), seemed like the best candidate to carry out the job.
Additionally, Hobbits in general seem to be one of the most incorruptible races in Middle Earth.
Well, I always thought it was a combination of how centered hobbits are - they’re very down with what they are - how powerless they are in general.
The Ring grants power and temptation based on the power and ambition of the bearer. In Gandalf’s hands that would be a problem. He’s moderately ambitious - in terms of wanting to achieve things - and hellaciously powerful.
In a hobbits hands what’s the Ring to tempt them with? Better carrots this year? Maybe finally mastering that darn recipe for ale? I imagine it would be frustrating for the Ring.
He didn’t. Gandalf is an emissary of Tolkien’s mythological stand-in for God. Sort of like Jesus (There is a good case for explaining Gandalf, Aragorn, and Frodo as different aspects of Jesus on earth). His purpose is to unite and strengthen people to fight against evil (Sauron et al). To do that he always had to take risks that he would fail. And often he did fail.
Frodo was a great gamble on Gandalf’s part, but remember he had known Frodo since he was very young, and had been assessing his character for the best part of fifty years.
Tolkien also was, for lack of a better term, a pastoralist. He though the best folk, those with the strongest moral fiber, were simple farmers, rooted to the earth and not caught up in international intrigue. That’s pretty much what Hobbits were. Most Hobbits, however, wold be loath to leave the Shire. Hence Frodo, who just so happened to be in possession of the ring already.
Gandalf had no such notion that the ring would be safe, long-term, with Frodo. The ring was dangerous. There are a limited number of Maiar, and a nearly endless supply of hobbitses. So if the ring’s gotta be handled by someone they’d best be expendable.
Well, in the book he’d already had it for what, about 15 years without using it before Gandalf found out what it was, (the film’s even vaguer about the length of time, but certainly a while). His people generally seem pretty resistant and he’s the nephew and chosen successor of the only person who ever gave it up voluntarily. That’s a pretty good sign. He’s small and good at hiding. Another plus. He doesn’t seem to want it at all, but does take it seriously and is willing to travel. No red flags, at least.
Besides, in Hobbiton, what other options are there? He could take it off Frodo and give it to Sam or another hobbit I suppose, or get him to wait there while he tries to find someone else and pass it to them, but no-one else living save Bilbo (plus Gollum and Sauron I guess if he counts as living) have ever held it, so who else is guaranteed to be safer? Frodo doesn’t even know how to use it, aside from turning invisible, he’s not going to put it on and instantly become a powerful foe.
Basically; no better options to hand. The plan was to meet up at Bree (film version, to be accompanied the whole way in the book) after all, and just going Hobbiton to Bree, at the edge of the Shire, under normal circumstances wouldn’t be a risky or stressful journey. Gandalf was planning on being right there to keep an eye on him.
Also, as noted in the OP, the original plan was for Frodo to take the ring to Rivendale where it presumably would be given to someone else to destroy. It was only after Frodo volunteered to take it to Mordor that Gandalf saw the wisdom in letting him keep it for that purpose.
The movie made a big deal about Gandalf’s caution with the Ring, mainly to punch up the stakes, I’d say. I put it down to deviating from the written work to make for a better movie, kind of like Arwen substituting for Glorfindel in getting Frodo across the river. In the book, he had no trouble at all handling it (but did not wear it). Tom Bombadil* not only had no issues about slipping it on, he didn’t even turn invisible.
Consider the alternatives. Who or what could he entrust the Ring to ? If you throw it into the sea or bury it in a rockslide, it’s just kicking the can down the line and losing any decision power over its ultimate fate (because the Ring wants to be found on the one hand, and Sauron’s scanning for it on the other one). Maybe he could have hidden it on Arda but the Valar would have frowned on that, and you do not want to make beings with absolute godly powers who happen to also be your boss frown. So, no hiding it out of reach.
No keeping it on his person either - he’d be too tempted to fuck with it and was scared out of his mind when Frodo offered.
So who to entrust the Ring to ? The Elves ? They’re the only ones not yet corrupted by the Ring, and they’re super powerful - but Galadrielle’s speech to Frodo demonstrates how bad an idea it is. Too powerful, too “alien” in their thought processes.
The Men ? Yeah, no. Boromir was the very best of men, and he didn’t even last a small trek without losing his shit.
The Dorfs ? Thorin was driven insane with greed by the Arkenstone which as far as we know is just a very shiny rock. Plus there’s the whole Doom of Durin thing. These guys are the poster boys for self-destructive greed without a thought for the long term. No.
And then there’s the Hobbits. Who are so powerless and unambitious few people even realize they are a thing, and they own land and everything so that’s a plus on the hiding part ; and Bilbo spent something like 50 years with the Ring in his pocketses without using it for anything more nefarious than hiding from his in-laws. But then you’ve got Smeagol, who got corrupted in a hot minute. So while there’s demonstrably nothing in-built to Hobbits that protect them from The Shiny, Bilbo’s bloodline does seem well suited, in the short term at least.
Plus if you ask me, he got off on plain trolling Elrond’s council.
Gandalf is nothing like Jesus. If you want to make that comparison Gandalf is akin to a minor angel (Maiar). The Valar would be akin to Arch-Angels (akin to the angels Michael and Gabriel). Melkor would be the devil and Sauron a fallen Maiar (albeit probably the most powerful Maiar) corrupted by Melkor.
As to the OP Gandalf could not know for certain but Gandalf was very wise and well understood Hobbits having visited them many times (not to mention his time with Bilbo).
Hobbits were not all immune to the ring (or at least resistant to it). The most obvious case of this being Smeagol (aka Gollum). Smeagol was of a different branch of Hobbits (there were three) but they are all related. He was pretty much instantly corrupted by the ring and killed his cousin almost immediately after seeing it so he could possess it. Then we have Samwise Gamgee who clearly has a problem giving the ring back to Frodo after only carrying it for a short while.
Still, the ring has a hard time with Hobbits. Yes it got Smeagol but then, instead of getting the ring back to Sauron, Smeagol hid in a cave for a few hundred years coveting his precious. When finally someone else came along and the ring swicthed owners it was another Hobbit who sat in his Hobbit-hole with the ring for a few more decades. The ring could corrupt them but had a hard time controlling them.
Gandalf was able to get Bilbo to give up the ring willingly (albeit with a lot of prodding from Gandalf). The only person to have ever done so. Gandalf knew Frodo, he was of the same family and figured he probably would have a similar resistance to the ring (and he did). Also, there was really no one else for the job. Gandalf knew any other race would be corrupted tout suite.
Part of why the various Hobbit’s - inluding smeagol - were not corrupted as men (or elves) would have been is simply because they had no desire for ‘power’ - Yeah, Smeagol liked shiny things, but it seemed Smeagol simply wanted the ‘thing’ - not the power that it presented.
In the heart of Mt Doom - where it was arguably at its strongest - and Frodo was at his weakest.
Keep in mind that Smeagol was never a good person. Even before he got the Ring, he was disposed to treachery and cruelty. And even with such raw material to work with, the Ring still never completely corrupted him: If it had, he’d have been at the head of a mighty (but not nearly mighty enough) army marching with righteous fury on the gates of Mordor, where he could be cut down and the Ring retrieved by its owner.
But that was a necessary part of the plan all along. And Frodo wasn’t able to complete the task on his own. Of course, sparing Gollum’s life along the way was also Frodo’s doing and that act of kindness save the world in the end.
Note that hiding the Ring longer wasn’t an attractive option. Sauron’s armies were more than capable of conquering and destroying the few remaining parts of the continent not already under his control, even without the Ring on his finger. The mere existence of the Ring somewhere in the world was enough.
Destruction of the Ring was their only hope. It had to be brought to Mount Doom, which meant someone had to bear the burden.