God's loyal angels are called "holy" and "elect."
"the Son of man also shall be ashamed of him, when he cometh in the glory of his Father with the holy angels" (Mark 8:38; Rev. 14:10-12).
"I charge thee in the sight of God, and Christ Jesus, and the elect angels" (1 Tim. 5:21).
(both quotes are taken from the ASV)
Concerning 1 Tim. 5:21, Gordon Fee observes:
"The inclusion of the elect angels is unusual and serves to intensify the solemnity of the charge. Elect may either refer to the angels as the chosen ministers of God who carry out his will (Bernard) or serve as a contrast to the fallen angels (Kelly), probably the latter, given the context of judgment" (Fee, Gordon. 1 and 2 Timothy, Titus. P. 131).
When Jesus speaks of TWN AGGELWN TWN hAGIWN in Mark 8:38, he evidently means that the angels are "holy, set apart, sanctified, and consecrated" to God (Zodhiates). While we want to avoid what James Barr called "illegitimate totality transfer," I think that the aforementioned definitions of hAGIOS overlap with one another. The lingual sign HAGIOS points to the faithfulness and sinlessness (IMHO) of God's loyal angels. This holiness is derived, however, and not self-caused as is the holiness of God.
In the Similitudes, we also read:
"But you, having been strengthened by the HOLY Angel,and having obtained from Him such intercession, and not being slothful, why do not you ask of the Lord understanding, and receive it from Him?" (Sim. 5.4.2)
Thus, even in the patristic writings, the angels are recognized as being "holy."