The billboard was launched with a countrywide ad campaign and will remain posted until the end of July, The Denver Post reported Monday.
"The religious right is not a majority, but it has a strong voice and a lot of influence. We want to uphold the separation of church and state," said Michael Lee Smith, spokesman for the Freedom From Religion Foundation.
Smith said the billboard is expected to cost nearly $3,000 for foundation supporters in the area to keep it up.
Critics say the foundation is ignoring the influences organized religion has on society.
"I always support their freedom of speech," council director Jim Ryan said. "But the foundation is failing to acknowledge all the moral values and work for the common good that has grown out of faith traditions."