A good first step in atheist activism is to check in with organizations that have common cause with atheists. The following secular groups are working to stem the attacks by Christian nationalists against democracy and civil liberties.
International
Atheist Alliance International
Atheist Foundation of Australia
Association of Brazilian Atheists and Agnostics
Atlantic Atheist and Agnostic Association
Atheists of the Czech Republic
Internationaler Bund der Konfessionslosen und Atheisten
Humanist Association of Germany
Atheist Centre – India
Humanist Association of Ireland
Humanist Association of Northern Ireland
Freedom – Atheists in Israel
Atheistic Association – Netherlands
Norwegian Humanist Association
Humanist Alliance Philippines International
Iniciativa Atea – Spain
Trinidad and Tobago Humanist Association
USA Nationwide
Secular Coalition for America
American Civil Liberties Union
Freedom From Religion Foundation
American Atheists
American Humanist Association
The Secular Web
Secular Student Alliance
Atheist Republic
Military Association of Atheists and Freethinkers
State and Local
Atheists United of California
Secular Hub of Colorado
Iowa Atheists and Freethinkers
Iowa Faith Leader Coalition (about the group – not the web site of the organization)
North Texas Church of Freethought
Washington DC Secular Humanists