An equipotential cross with open centre, not filled with colour. Unlike the icon similar to this [U:271B], it's drawn with bold lines. That's why it attracts more attention when placed in a text or a post. 

Heavy open centre cross is rarely used as a strictly religious symbol. Many believe that it rather looks like a schematically drawn flower with four petals, not a cult sign. You may come across it in texts or posts, where it's used as a dingbat - a kind of decoration or embellishment for bullet lists, titles, important footnotes, quotes. Apart from that, the symbol is often applied in nicknames and other non-religious contexts.