Secularists do not desire to drive religion from the public sphere. A society's expression of faith is one that defines the fabric of that society. How our society treats those who are spiritual in any form defines the people of that society.
But also consider this: it is important that while individuals are free to express their faith, it's also important that your faith no dictate your decisions in public policy. Using faith to justify public policy exclusively is harmful to society, and blurs the veil that separates church and state. The church and state argument derives from Thomas Jefferson's belief that there should be a wall between the government and the churches. He saw the corruption that existed in the UK where the Church of England is a public church, funded by tax payers. The government and church both engaged in highly questionable political and religious interactions, trading power and money for offices of worship and spiritual guidance.
Consider also the 500s through the 1700s, where for 1200 years, the Catholic Church actively interfered or in some cases even outright ruled nations using faith to instill fear, suppress the spread of education, and demanding full and unquestioned fealty, or face persecution by the Church. It was a dark age in Europe where science was dismissed and condemned, and questions glared at with disgust. The only people who benefited from that system was the Church, and it was the state that suffered at the hand of the Church.
So when I say I favor the separation of Church and State, I base that view on the history of interactions between church and politics. It's important that we understand that the separation of Church means keeping the judgments of churches out of governance, but not your expression of faith from guiding you as an individual. Christianity extols the values of ethical conduct, righteous administration, and servant-based leadership. It begs people to fight injustice, to expose the dark areas, and to bring judgment to those who commit wrongs.