If you read the note at the
bottom of the Wiki map, you'll note the secret door is Level 35. With the 3%
bonus / malus of finding secret doors, your getting a 51% penalty to your passive change to find a secret based on your character being Level 18. Since you appear to a be a warrior, you might have a pretty low passive detection rate. (You can check your [passive chance to find secrets on your character sheet.) Thus, you likely have a negative chance to find that door -- which you you can't discover it passively. You'll have to use some few Scrolls of Detection -- each one has a 75% chance to find that door. So bring a few.
What do you mean the gate to hell? The teleport near the Abbot? That's an one way out of the
Hellish Cave not into it. You'll have to get past the secret door discussed above and go down to the Hellish cave.
The gate in the Abby on the north side of the first floor is opened by talking to non-ghost ghost of
Saint Admus. He'll ask three questions that you must answer correctly. The answers are found by talking to a NPC in Fogas, and reading the book found in the Abby. Or you can just use the
quest Wiki.
I'm anticipating the tricky part to the
Demonic Lawyer might give you a headache. As the quest Wiki states, you'll have to talk to the Demonic Lawyer (not fight), ask about the Abbot's uncompleted contract. (Note: you'll have to run across some burning floor to get to the Demonic Lawyer.) Only AFTER talking to the Demonic Lawyer can you find about about the Abbot's loophole by reading the Abbot's journal. You can skim the journal before talking to the Demonic Lawyer, but your character isn't that interested in reading about the diary of madman. After finding about the loophole, go talk to Saint Admus to get rid of the loophole. Than you can kill the Abbot.
Hope that helps.