Justice has to have strong foundations if it is to remain resilient in the face of the world’s many threats. Secularism denies having any ultimate, absolute foundations. Therefore it won’t be able to sustain any commitments to true justice. Any foundations it has are borrowed from other value systems, mainly Christianity.
Here’s “postmodernist bourgeois liberal” Richard Rorty in a revealing quote:
[It is] part of our tradition … that [a] stranger from whom all dignity has been stripped [should] be taken in and re-clothed with dignity. This Jewish and Christian element in our tradition is gratefully invoked by freeloading atheists like myself who think that metaphysical debates are futile.
This is a quote scholar Robert Kraynak makes good use of in this interesting article.