«هُدًى لِلْمُتَّقِينَ»
Guidance for the God-fearing.
«هُدًى» is indefinite. In this context the indefiniteness can magnify the excellence of the guidance: the expression does not merely hide the name of some familiar guidance but leaves its quality open to a more elevated estimation.
Indefiniteness does not automatically create grandeur wherever it appears. The subject matter, a context of praise, and the broader statement guide the listener toward an exceptional degree or worth. That context is what grounds the rhetorical reading.