Alan Jacobs quotes C.S. Lewis, on the 'decline of religion' in modern society here.
When it ceases to be respectable or mandatory, when participation in religious institutions is no longer expected, then we can finally observe who really takes Christianity seriously...and that might not be such a bad thing.
Its easy to coast along under a vague and general societal and cultural Christian ethos, a bit like how its easy to roll down a gentle slope. But the Christian life described in Jesus' and the Apostles' teaching, and as experienced by the church through most of its history, and still experienced by the majority of Christians in the world today - in places like China, the Middle East, or Islamic Africa, for example - is more like trying to roll up a steep hill: all kinds of forces are trying to pull you down backwards.
The true test of genuine Christian faith in our secular society is not in advocating for the return to vague and culture-wide Christianish practices, participated in by everyone - Christian, nominal and non - like the return of prayer in school, the return of the 10 Commandments to court rooms, or for politicians to pray to a generic deity when opening a legislative session. Rather, the test of true Christianity is steady, humble yet bold, public witness in word and deed to faith in Jesus Christ and submission to his lordship. Individual Christians and the Church together must simply practice our faith in the midst of a culture that not only no longer recognizes once generally accepted Christian practices, but is frequently antagonistic towards them. We ought to continue to practice our convictions amidst such a culture rather than wishing to go back in time or expecting the rest of the secular society to respect or reinstate previously observed Christian practices that used to be part of the generally accepted broader culture.
The West is increasingly post-Christian, which is a far more challenging mission field than merely non-Christian or pre-Christian. The way modern post-Christian society views the faith is that it has been tried, disproved or found to be unnecessary or irrelevant, and rejected. This has the effect of inoculating culture against the Christian faith. So for Christians to long for or try to politically reinstate a bygone era of nominally cultural Christian practices is not the way to witness to the gospel in a post-Christian society. Rather, effective witness will be when convinced and sincere Christians live lovingly but boldly according to Scriptural authority and the lordship of Christ, reaching out to their neighbours and co-workers who are struggling to find transcendent meaning in a self-centered and dehumanizing culture. In such a culture, people living life consistently in relation with God and each other, looking not only to their own self interest, will look so very different.