There seem to be infinite styles and themes in Holy Cards. Most of my cards are in a chalice with flowers style, many with roses.
These cards are usually very highly decorated. There's almost always gold edges and accents, and the flowers are always lush and fresh.
The fonts are frequently amazing; very detailed in design or layout. There may be decorative accents around some letters, various colors of capital letters, and each line is often underlined with tiny scroll work on the ends.
The border designs may have an art-deco look or fabulous colorful scroll work that put all of our recent digital brushes and rubber stamps in the pale.
The nicest cards I've found have been from eBay. My favorite dealer there is ICS Vintage Store in France.