I'm not talking about a priest kit, but rather choosing a deity.
A deity is the god or lord or whatever you call it that your character worships, and for that worship they grant you access to specific spheres and priestly powers. A kit is a specific type of priest character, and this is not what I'm talking about.
You need to review one of the several deities books for 2nd edition and choose a deity to worship. The list of books to review includes Faiths & Avatars, Powers & Pantheons, and Demihuman Deities. All of these books list the different deities in the Forgotten Realms, and under each deity you'll see the specific spheres and powers a specialty priest gains.
Let me know if this is more confusing than helpful, and I can help walk you through it.
EDIT: I realized that you quoted F&A in your post, so I need to edit mine. Some days I can be...stupid.
Anyhow, even if you don't qualify to be a specialty priest of a specific deity because of multi-classing, you still need to pick a deity to worship. That will still determine your access to spheres, even if you can't get the priest powers for not being specialty.
Hope that helps!