An "activist judge" is a judge who petulantly refuses make everyone else live under the rules of -your- religion.
The constitution says "No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws." Note that it doesn't say "heterosexual, white, christian person". There are no 'activist judges'.
Freedom of religion applies to everyone not just Christians.
The "Equal Protection" clause of the 14th amendment applies to everyone not just white male heterosexual Christians. When courts make rulings acknowledging these simple truths Republicans call them activist judges.