The heist happened tonight.
The party did a lot of scouting, first at a wedding, then by sneaking around on the roof, and by using magical disguises to make friends with some of the priests, and by speaking with animals to learn from the rats and birds that lived in and around the temple. Over the span of a few weeks they made their plan, and it went off astoundingly well.
Five PCs: human barbarian, changeling ranger, githyanki rogue, dragonborn alchemist, and tabaxi fighter.
In preparation, the alchemist made a lot of potions, and a bag of holding, and a rope of climbing. The rogue made a duplicate clay tablet which they could swap out for the one they were stealing.
First, the party got a little lucky when a fellow NPC criminal attracted the attention of a paladin of the temple, so when the paladin showed up at the thieves' den asking questions, they PC, lol, ratted out their peer, forcing her to flee into the night. Then when the paladin chased after her, the PCs ran along as 'help,' and when the paladin cornered the woman, they ganked the paladin and his guards, stripped the paladin of his gear, and had the changeling PC infiltrate the temple.
Second, that same night the barbarian PC recruited some random petty brigands with an easy pay-day: all dress the same with masks and hoods and spears, follow him to a job, and then after the job was over, everyone scatters to make it harder for the authorities to catch any of them.
Third, instead of sneaking in at night and having to deal with golem guardians, they had the changeling-as-paladin invite two of the temple priests (out of a total of 16 show live there) to lunch. After luring them away, he doubled back and met up with two other PCs who had used potions the alchemist made to change their appearances, and donned priestly attire acquired from some weaver friends of theirs. A minor NPC ally started a fire in a supply building next door to the temple, and the 'paladin' sounded the alarm, calling for priests to come help use water magic (since they worship a sea goddess) to stop the fire.
Fourth, the rogue (one of the two PCs disguised as a priest) used the commotion of people rushing to fight the fire to open a servants' entrance through the kitchen, through which the barbarian and his look-alike gang rushed in. They bee-lined for a side room which held a legendary spear sacred to the temple. A second paladin happened to be in that room, having a hunch that the fire might be more than a coincidence. He saw five men rush in with spears and swore he would kill them all and feed them to the sharks. The brigands with the barbarian decided they could easily take the guy with their superior numbers, so they attacked . . . while the barbarian just yoinked the sacred spear and ran away.
Fifth, the alchemist (the other PC disguised as a priest), pretending not to hear the sounds of fighting in the spear chamber - and the barbarian running away - simply walked up to the magical pool that had the table they were trying to steal, waited until no one was looking, and opened the bag of holding, which contained their tabaxi fighter - who had previously drunk a potion of alter self to be able to breath water. He threw the bag into the pool, with the rope of climbing attached to it by a string. The tabaxi swam out, used a crowbar to pry the tablet free, put it in the bag of holding, put the fake tablet in the empty spot, then climbed into the bag of holding himself and tugged the rope as a signal for the alchemist to pull him back up.
At that moment, the real paladin came out of the spear chamber into the main chamber with the magical pool, in pursuit of one of the brigands the barbarian had hired. The brigand was running, and the paladin struck him down with a hurled spear. The alchemist disguised as a priest pointed and said he saw another thief go that way (which was not the way the barbarian really went), and the paladin sprinted off in pursuit. The alchemist then reeled in the bag with the tabaxi and the tablet, used a cantrip to dry himself, and simply walked out of the temple.
The whole theft took less than two minutes from when the alarm was sounded for the fire to them escaping with their prize. As far as anyone knows, the fire was just a distraction for a gang of masked men to run in and steal the spear, and so no one suspects anyone did anything with the magic tablet. And in a couple days the barbarian - who in his 'day job' is allied with the queen, will hand over the spear, making it seem like he got it from the paladin (the one they murdered), whom he'll cast as the real thief. This will endear him more to the queen, and put a tidy bow on the whole heist so no one thinks to investigate any further.
Among potential 'loose ends' is that the paladin they're pinning it on won't have a body to turn over to the temple. In this setting, bodies that are not cared for after death - either by ritual cremation, or by burial and intermittent libation offerings - can rise as undead. The barbarian can hear ghosts, and while he was willing to murder a paladin for his mission, he insisted on respectfully burning the body.
But the real complication is that, after the party linked back up and head for their hideout outside town with their stolen holy tablet, they took a small boat over a canal. Now, the tablet they stole says something like:
'In keeping with the commandment that calls waters from afar, the priest of this temple shall have the power to name what beings and objects are permitted to travel between those waters and the waters around this dictate, and which shall be hedged at a distance of twelve cubits.'
But after they moved the tablet outside the temple, the "this temple" became undefined, and so did the "what beings and objects." And so the next time they entered a body of water - a canal - the tablet basically glitched. The instant one of them tried to step onto dry land, the whole shebang - PCs, boat, tablet, and even the ferryman - all were shunted into an extradimensional space - a dungeon that I'm calling The Consequence, which is fed by all sorts of disruptive interactions and unintended, well, consequences of different magical tablets.
So next session, the heist switches to a puzzle dungeon. What the hell is going on in The Consequence, how do they get out, and how can the stolen dictate help them?