The “scarlet A” is a symbol used by atheists to recognize each other, started by the Out Campaign. It’s a red capital A from the Zapfino typeface. The Out Campaign is a is a public awareness initiative for freethought and atheism in the United States, founded by Robin Elisabeth Cornwell, and endorsed by Richard Dawkins. From Wikipedia:
The campaign aims to create more openness about being an atheist by providing a means by which atheists can identify themselves to others by displaying the movement’s scarlet letter A […] an allusion to the scarlet letter A worn by Hester Prynne after being convicted of adultery in Nathaniel Hawthorne’s The Scarlet Letter. It encourages those who wish to be part of the campaign to come out and re-appropriate, in a humorous way, the social stigma that in some places persists against atheism, by branding themselves with a scarlet letter.