I think that Christians who have appeared on the ABC's Q&A are helping to break down a popular misconception of the phrase "separation of Church and state." They're showing that there is an interaction but not a separation between the Church (Christians collectively) and the public sphere. What I'm describing here is not how my faith affects me as a cultural creature, but how my faith affects other people and their culture. Collectively it's the question of how we shape the shared public sphere. It's the practical question of all human civilisations; what should the shape of all of us together look like?
The public sphere (politics, society and culture) should ideally encourage human flourishing. This means the moral and aesthetic values of life, liberty and truth. These are or at least should be the guide and telos of our earthly civilizations. They spring out of a Christian worldview but they're also naturally attractive for a shared public sphere. This is why ideological pluralism flourishes in most liberal democracies, compare for example India with Saudi Arabia, or even nominally Communist China with North Korea. Thinking about the public sphere flows out of two aspects of Christian theology. Firstly, life, liberty and truth foreshadow the shape of our eternal life with God and secondly not everyone who belongs to the visible church is part of the invisible church* and it's not our job to sort out who's who (Matt 13:24-30), we have to get along together on this side of eternity.
There are several practical benefits from thinking like this. Evangelistically, having the public sphere arranged like this prompts the question of why those particular ethical values. Sociologically encouraging human flourishing benefits everyone, secularists and co-religionists. Additionally, just the concept alone of a robust shared public sphere, encourages the discussion of ideas which in turn lays a strong foundation for ideology and therefore theology.
[A picture of Thomas Jefferson who first articulated the concept: "separation of church and state." But interestingly not quite as it's commonly understood.]
Sharpening of ideas: h/t Will Briggs ;-)