The promo scheme is:A fighter can become a crusader or a mercenary
A crusader can become a paladin or a ranger
A mercenary can become an assassin or a gladiator
An initiate can become a healer or a scholar
A healer can become a priest or a druid
A scholar can become a lich or a mage
You cannot legitimately cross from one of these lines of promotion to any other; for example, your scholar cannot ever be promoted to a priest. The only way to do that is to use a hex editor to edit your savegame file.
One last thing - if I were you, I would avoid the crusader, paladin and ranger. They are weak compared to any of the other classes. In fact, not so much weak as pathetic.