I'd like my players to take their characters' in-game religions more seriously, along with that of NPCs, rather than treating the various gods like spell vending machines and clerics as no different from wizards. Any tips, that aren't too heavy handed? I've broached the subject out of character, but haven't achieved what I'm hoping for.
Basically, the characters treat any show of devotion to the gods as a joke. Obviously I'm not offended on behalf of these fake gods, I just want a different tone.
One way to do this (and a way that mirrors how religions work a lot of the time in the Real World): the faithful control the politics and behavior of every significant organization or civilization the party comes across.
To make it very Medieval Europe: the gods are all organized into a single Church, and that Church is very wealthy and influential. The people go there weekly. The gods inspire pilgrimages to holy sites. Saints are mentioned more than any other heroes. The leaders of the church are the power-brokers in the places where they live, and even in the broader kingdom: to be a true king, the Church must recognize you.
In play, this manifests as the Church affording special privileges to the religious characters. In order to meet with a particular NPC, they need to show up at church. The church's leaders are patrons of the party, and the missions they undertake are specifically for the church first and foremost. Every meal is opened with a prayer, everywhere the PC's go. Every oath and contract is sworn with the names of the gods. Money is printed with holy symbols.
Make the gods important to the NPC's of the world first.
If the party expresses in-character irreverence, make sure the NPC's react appropriately - with fear, with disdain, with ostracism. Influential NPC's become unfriendly, even hostile.
You might also get at the heart of the matter by linking each character to the Church in some way (as would be the case being raised in such a religious setting). Ask each player to describe the Church that they attended in their homeland. Link it to their class, or their background. (You
might consider adopting training rules, and making sure trainers are members of the church)
Of course, get player buy-in for this. If you want them to be "respectful" characters, talk to them about it, say it's part of the tone you're going for. If the players aren't on-board, then no amount of anchoring is going to really change that, and they might just rebel.