As a subcategory, liturgical art serves a very specific purpose: to teach and inspire in the Christian faith. Therefor it must be clear to the onlooker who has at least a modicum of understanding of the faith. In addition to being clear, it must use caution when using symbolism when it is not in keeping with the tradition that has been established to help get across meaning. That does NOT mean that new modes of symbols cannot be employed, but they must be clear.
Using established symbols in a new fashion may be a cause for confusion. For example, a numbus or halo that is specific for the Godhead placed on another person may confuse the message. Ot using something whose meaning only makes itself clear when explained by the artist is not doing its job. For example, there is a window in one church in which a couple is archaically involved in amorous behavior. If you didn’t know that the a little red circle in the upper corner of the piece was intended by the artist to mean “Don’t” thereby signifying the commandment “Do not covet they neighbor’s wife,” then you it might lead the observer to wonder what exactly is going on here. Liturgical art is somewhat less free than general categories of art. BUT this does not mean that it cannot challenge, evolve and be enormously creative and never, ever - unless under dire circumstances, mass produced and bought in a catalogue. (Then why not just tack up the page from the catalogue?)
Liturgical art lifts up, it inspires, it ennobles, it challenges, it should dazzle with beauty and/or stretch the intellect. Before such a piece, even a non-believer should be able to sit before it, receive a message, and be able to pour out his heart.