Near the end of the game, several idols will be unavailable for fast-travel. You'll have to defeat the remaining bosses and clear the game to be able to use them again.
You can fast-travel freely between any discovered Sculptor’s Idols, including the Hirata Estate located in the past. Sculptor Idols serve as the game’s checkpoints. You should prioritize communing with new ones as soon as you discover them. Take note that idols are not entirely safe zones; there are idols with enemies nearby and if you aggro them, they can still kill you even if you commune with the idol. Fast-travelling has the same effect as resting; your vitality, resurrect charges, and Healing Gourd is replenished while normal enemies will be respawned.
The Homeward Idol is a non-consumable item that will conveniently take you back to Dilapidated Temple or the last communed idol. Don’t forget to set this to your Quick Items so you can use it instantly to quickly return to a safe location. This item can be used as often as needed, even while in the water.