Internet Explorer 7 is out soon, and it will be delivered as a priority via automatic updates, so the take-up will be pretty quick even if people do not consciously go to download it. Because many sites were developed for the earlier quirks of Internet Explorer, it is likely/possible that some sites will lose functionality or not work at all.
The good news for churches using wordpress for the content management of their site, is that it is standards compliant, and theme developers are strongly encouraged to follow standards based design in their theme creations. This means that if your church already uses wordpess, you will probably have no problems when users adopt the new browser.
However if you have a custom designed church website, it is important that you test all its features with a copy of IE7 beta, so that any possible changes that are needed can be incorporated before people have problems accessing your church website.
The IEBlog has some helpful tips and links on becoming ready.