Make use of your starting favour with the Pope. Begin a Crusade against Jerusalem. Try to get there first. Take Jerusalem (along with the Pope's reward) and expand across Africa. From this point onwards, try to maintain alliances, don't bribe, and only attack if attacked and/or betrayed (for trustworthyness, it really makes diplomatic deals easier). Take part in any crusades that are called, and try to get to the target first. This way, although slow, allows military build up, and is the easy way to victory.