The answer that Koppelman comes up with is that a). religious moral arguments aren't any more convincing and b). because you can, can, can. This is called "Naked Strong Evaluation" or the willingness to assert something without the protection of a metaphysical argument to back you up. In other words secular morality requires only that you be brave enough to go out on the wire without a net and be willing to withstand all of the nihilistic counter arguments that state that without religion all is permitted.
The essay also has some interesting things to say on how this ethical question was created in the Protestant Reformation and the attempts to remove paganism and superstition from the church:
The movement that culminated in the Protestant Reformation began in the Middle Ages. There were repeated efforts by the church, first to reform its own practices and later to restrain as idolatrous the veneration of saints’ relics, magic, miracle-mongering, and dancing around the maypole. The Reformation radicalized this move by abolishing this tension and inaugurating the “priesthood of all believers.” Ordinary life—work, play, sex—began to take on sacred meaning. The Christian virtues were no longer those of ascetic monks; an ethos of personal responsibility and self-discipline became available to everyone.It's an interesting line to pursue. The idea is that as long as religion is grounded in magic and wonder, God is not accountable to human beings. His goals are his own. But as soon as religion is stripped of its madness and irrationality, Christianity becomes constrained by the logic of its own moral order: saving souls, defeating evil, and living by God's plan. Once it falls upon the individual to flourish or fail, the differences between religious and secular life become a matter of personal preference.
This attempt to bring Christ into a world that had become desacralized inspired a new focus on the world. Human beings now had to inhabit the world, writes Taylor, “as agents of instrumental reason, working the system effectively in order to bring about God’s purposes; because it is through these purposes, and not through signs, that God reveals himself in his world.”
This disengaged stance toward a disenchanted world became the moral basis of the new scientific method. Technological control of the world became yet another way of doing God’s work, benefiting the human race in accordance with his plan. The highest goal was understood to be “a certain kind of human flourishing, in a context of mutuality, pursuing each his/her happiness on the basis of assured life and liberty, in a society of mutual benefit.”
The “this-worldly ethos” eventually made it possible to cut loose from religiosity altogether. Once “God’s goals for us shrink to the single end of our encompassing this order of mutual benefit he has designed for us,” it is easy for God to drop out of the picture completely. The goal of order becomes simply a matter of human flourishing, and the power to pursue that goal is a purely human capacity, not something we receive from God.
