Or you could actually explain it. What was worth the cost of becoming secular. You're relying on an old Protestant author to say why religion needs to be removed from public life. We actually live in the present and can see the results of such secular reasoning. To me, it doesn't seem worth it.
Roger Williams was a staunch Calvinist with a deep believe in Election. He saw government-mandated or government-rewarded Church membership as putting the non-Elect into the pews, and because the non-Elect prophetically outnumber the Elect, that meant most Church "members" would thus be unbelievers. Worse, if the state conveys a social advantage to Church membership, the people who desired that social advantage would be the very people most apt to join the congregation.
Roger Williams examined the history of Church entanglement with the state from Constantine to the Thirty Years War, and also with an eye toward the then-current religion-based social unrest that would soon erupt into the English Civil War. He discovered that when earthly states mandated Church membership, or awarded social privilege to the Church, that it inevitably resulted in social conflict between Christians (or people professing to be Christians). It was always Christians who eventually suffered from state patronage of the Church.
Williams also saw that historically when the state became the patron of the Church, the Church became obliged to support state actions, unable to stand objectively indifferent. It's like being the chaplain of a cruise vessel. The chaplain has no say over what happens on the ship or where the ship is destined...but if there is a storm, they expect the chaplain to pray them through it. In Williams' view, the Church should never become the chaplain to the state. The problem, he saw, is that in this fallen world, this world of tooth and claw, it's impossible for the state to operate in a Godly manner. In order to maintain power in this fallen world, the state must use the weapons of the fallen world.
He believed that the best thing for the Church was for government to keep its hands off the Church, and for the Church to refrain from government entanglement. Williams founded the colony of Rhode Island as the first colony with total freedom of religion, including for atheists and Muslims, so that the members of the Church would be those brought into the pews by the power of the Holy Spirit, not the sheriff's gun.
It was from Williams' writings that Thomas Jefferson cribbed the line, "...wall of separation between Church and State," and it was Williams' Rhode Island philosophical descendants who blocked ratification of the American Constitution until the First Amendment was written and ratified. I know Down Under y'all don't care very much about the First Amendment, but it's an important thing for Americans...and its purpose was to preserve the purity of the Church.
Has it made your country less violent? No, it regularly starts wars and funds foreign conflicts, not for religion but for secular liberalism. Has it made the social fabric stronger? We're more atomized and disconnected from each other than ever before. Has it made Christianity stronger? No, instead it's made churches conform to the dominant secular liberal ideology. I don't see how it was worth it.
No, the problem in the US had been that the American state
did eventually become the patron of the Church...and the Church
did end up endorsing every unGodly act the American state has used to consolidate its power in the world.
When we observe the steadfastness of the Church in North Korea--which has grown 10-fold since the 1990s--it's obvious that the Church in America has
never been subjected to a force that should have caused it to conform to American social pressure.