The horizontal stroke through the middle of a handwritten 7—common across Continental Europe, the UK, and many Commonwealth countries—is not a religious symbol. It's a disambiguation mark.
In many European handwriting styles, an uncrossed 7 resembles a 1 (especially with a serif or upstroke) or a sloppy 9. The crossbar—technically a virgule or stroke—makes the digit instantly distinguishable at speed.
Fonts rarely include a crossed 7 (exceptions: some monospace/technical fonts like Consolas or Inconsolata for legibility in code). The cross is a handwriting convention, not a typographic one.
So: the cross on a European 7 means "this is a seven, not a one." Nothing more, nothing less.