It means that Islam is not true. It means that only Christianity is true. There is no fellowship with darkness. Universalist philosophy, if accepted by the worlds' religions, would greatly reduce the focus of differences between them. It would greatly reduce the need for orthodoxy, for defining and preaching the true God.
Truth is at stake.
Universalism in itself is not a deception, just an error. I can not say, but even one is too many. I cannot help but notice that the secular world teaches tolerance, be-nice philosophy, and the belief that all will make it to heaven. Second chance redemption and a theology that teaches you'll make it no matter what is comfortable to the world's ears. Too bad the universalists agree with the world in this.