Authentication is not something you would normally set up yourself. You would talk through the options with the people providing the Moodle site and decide which is right for your context. Your three main options are

1) Create accounts for each person

This is the simplest method and is suitable for situations where you know precisely who should be given access. So for example if it is all curates or all people on a particular course. Accounts can be created manually, or you can create multiple accounts by uploading a spreadsheet. So if you have a spreadsheet with all the clergy and licenced ministers in your diocese then you could create an account for all of them in a few minutes. When creating accounts you are required to give just four bits of information (firstname, lastname, username and email), but you can add as much information as you wish (following GDPR rules of course). You also have the choice of creating passwords for people (which they will change when they first log on) or having Moodle email new people with an automatically created password.  

2) Allow people to sign up and access the site once their email address has been validated

This generally works well, but you will need to be prepared to offer support for people who do not understand instructions like “click on the link in the email that is sent to you”. In the typical diocese this is less than 1% of the people who sign up. Again the minimum information people need to give to create an account is firstname, lastname, username and email. However you can require much more, including dropdown lists to select parish etc. You should be aware that anyone who finds the web address for your site can create an account and access the site, so you need to give thought to security within the site.

3) Use some other service that people have signed into already to provide authentication

This would work well if all the people who might access your site have a diocesan username and password for say the Contact Management System. You could then use that system to provide a logon to the Moodle site. You could even connect to Facebook or Google if you wished. The standard protocol used is called "OAuth2". We use a special system to connect two Moodle sites together so that if you log into the first you can access the second without signing in.