There is no harder theological task then accurately describing the "holiness" of God. Various Greek and Hebrew words are translated as "holy" and each word can be used various ways depending on context. Holy can be mean "separate" or "not common" or "morally perfect" or "unblemished", etc. but none of these really does the job.
God's holiness is such that it creates earthquakes when His presence intensifies, evokes such fear in an individual that he or she instinctively falls to their knees and hides their faces (hence, our normal posture for prayer) and even strikes those dead who dare to dishonor it.
God is holy and there is no concept more difficult to understand until we stand at the throne.
More later here and from CCC this weekend as this subject is the second prong of my sermon on "The Lamb of God."
Hope to see you there.