Yes take the bag. Would be foolish not to.
You take the bag and equip it. Inside the bag you find a map to the dungeon.

You take the map and equip it.
Temple map mechanics:Moving in a dungeon is different than that of moving outside.
You can move any where that you want to, except for on a wall or on a door.

If you are on O - 3, you can move to any square inside of the current room such as N - 6 or M - 8, excluding doors (brown) locked doors (yellow) Treasure chests (dark brown) and boss fights / exits (red)
If a room is green, that means that the room is mostly safe and you have a very small chance to encounter a monster.
If the room is light gray, that means that there is a 50/50 chance of fighting a monster.
If the item that you interact with is bright red, there will be a 100% chance of a boss fight with valuable loot dropped. Also the only way out of the dungeon is through the dungeon boss.
To interact with an anomaly on the dungeon map, simply stand one square next to in in any direction.
To successfully open a locked door, you need a key item. You can find these by grinding/battling monsters. Each monster has a 20 percent chance to drop a key.
To interact with a chest or a door, simply stand next to the door or chest and request for it to be opened.