Belloc, writing The Great Heresies in the 30s, seems rather pessimistic about the future. But I think he sizes up the enemy pretty well. But as the enemy of secular modernism has grown and mutated, so has the power of Christ in His Church. This picture is merely an example of the "turning of the time."
Belloc says that "Protestantism" as a movement was already dead in his era, in that the great Protestant movements of England and Germany had begun to fade and sink. It's interesting that most "Protestants" today would not speak of "Protestantism," but Christianity or maybe Evangelicalism. Even fewer look to the magisterial Reformers - Luther, Zwingli, Calvin, Cranmer, et al.
Even the real Calvinists in the PCA aren't deeply interested Calvin or even read him. But Belloc had not foreseen the modern daughters of Protestantism, the energetic Evangelicals and Charismatics, neither of which see themselves as "Protestant." Also, Belloc did not foresee a renewal of Catholicism. Granted it has been bitter-sweet since the Second Vatican Council, but I think the Church militant has experienced a net gain in growth and influence. Could he have imagined the pontificate of John Paul II or a German pope reigning from St Peter's Chair? Could he have imagined that 4 out of 9 American Supreme Court Justices would be practicing Catholics? The spiritual renewal of Poland?
Sure, it is not a complete renewal and the Church is far from returning to her former place of influence over white culture (far, far away), but she is arguably in a position similar to the situation just prior to Constantine - this time it's not the Roman Empire, but the actual globalized world. The governing and economic structure of the earth is changing once again, and the Church has spread leaven throughout. Perhaps she may triumph again.
There is also a number in the equation that could turn the tables on everything: Islam. Either the Church will fight on two fronts, Islam and Secularism; or Islam will terrorize the Western world so terribly that the West will return to Catholic Christendom for refuge. I don't think secularism has the potency to hold up against Islam. Only Catholicism has a universal culture and philosophy that can overcome Islam's ascent in the world.