In what sense are angels elect? In 1 Timothy 5:21 we read:


  I charge you, in the sight of God and Christ Jesus and the elect angels, to keep these instructions without partiality, and to do nothing out of favoritism. 1 Timothy 5:21 (NIV)


My understanding is that a Reformed Protestant would use the word "elect" to describe Christians predestined by God to be saved from their sin, and brought into His kingdom . I'm not aware of any biblical evidence that angels have gone through that process, so it seems odd for Paul to use that word here.

How would a Reformed Protestant understand Paul describing angels as elect?
The 1657 Westminster Confession of Faith, which the Reformed Tradition subscribes to, indeed explicitly understand some angels as elect.
Quote from Chapter III (Of God's Eternal Decree) points 3 and 4:

III. By the decree of God, for the manifestation of His glory, some men and angelsf are predestinated unto everlasting life, and others fore-ordained to everlasting death.g
f I Tim. 5:21; Matt. 25:41.
g Rom. 9:22, 23; Eph. 1:5, 6; Prov. 16:4.
IV. These angels and men, thus predestinated, and fore-ordained, are particularly and unchangeably designed, and their number so certain and definite, that it cannot be either increased or diminished.h
h II Tim. 2:19; John 13:18.

There is a Reformed article on 1 Tim 5:21 which addressed these 2 questions:

Who are the “chosen angels” and what does their title imply?
Although the Westminster Confession of Faith includes a reference to “angels,” most Calvinists are unwilling to draw comparisons with men and angels in terms of Election and Reprobation. Why is that?

The article quoted John Calvin to conclude that

“because they were elect,” the angels were prevented from falling, perhaps by an Irresistible Grace, rather than being declared Elect on account of not falling.

To reconcile the difference in that &quot;elect&quot; humans are Totally Depraved (which cannot be for angels), the article quoted from a book Commonly Misunderstood Bible Verses: Clear Explanations for the Difficult Passages by a 4-point Calvinist Ron Rhodes (pp.255-256, emphasis mine):

“All the angels were originally created good and holy, just as God made and pronounced all His creation good (Genesis 1:31; 2:3). For God to create anything wicked would be inconsistent with His holy character. Jude 6 affirms that originally all the angels were holy creatures. God did not create Satan and the fallen angels (demons) in a state of wickedness. Though all the angels were originally created in a state of holiness, Scripture indicates that they were subjected to a period of probation. Some of the angels remained holy. Others did not--following Lucifer’s lead, they rebelled against God and fell into great sin. Once the angels were put to the test to remain loyal to God or to rebel with Lucifer, their decision seems to have been made permanent in its effect. Those angels that passed the probationary test will now always remain holy. Those who failed the probationary test are now confirmed in their evil state. This is the reason the good angels are called elect angels in 1 Timothy 5:21. They are not called elect because they sinned and then were elected unto redemption (they never sinned during the probationary period). Rather, they are called elect because God intervened to permanently confirm (elect) them in their holiness so they could not sin in the future. Good angels are now incapable of sinning. The lines have been drawn, and the lines are now absolute.”