What has this to do with the Netherlands? I think the following applies to all civs and not only to William (so this post is more or less off topic). Please explain why you link the Netherlands to faith in your post.
You're not always converted. Plus, you could use inquisitors to protect your cities from being converted. Or you could be an ****** and capture any missionary or GP you see. And the best option: buy GPs or missionaries to convert enemy cities. It's all gold bonuses, with the right beliefs. You could also go for happiness beliefs to get that extra golden age, but always watch out for the enemy missionaries.
I do understand that it could be hard to use faith early on, because initially you have to invest (almost) all of your gold and/or hammers to produce faith buildings, which do not give you a lot at first. You could also have invested in other buildings that give bonuses right away. But faith gives you some new strategy options.
An example: using faith to produce even more faith (through the holy site, for example) and then converting many, many cities (including CSs, that want your religion), you can gain serious advantages, through CS bonuses or follower beliefs.