Angels are not to be believed if they say what is false

Galatians 1:8 But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let them be under God's curse!

Paul is no doubt speaking of a hypothetical situation, although there is every reason to suppose a fallen angel preaching a gospel other than the one ... preached to the Galatians by Paul. As glorious and wonderful as angels are they are only to be listened to as far as they speak God's word.

Angels have a special commission to guard believers

Psalm 91:11, 12  For he will command his angels concerning you to guard you in all your ways; they will lift you up in their hands, so that you will not strike your foot against a stone.

These are the verses that Satan tried to misuse when he tempted Jesus in the wilderness. We are not to tempt God but we are to be confident that his angels will guard us in all our ways, if we are believers. They will make sure we come to no harm.

Spurgeon says that rather than having "one guardian angel, as some fondly dream" all the angels are active on our behalf. He calls them "the bodyguard of the princes of the blood imperial of heaven".

"It is down in the marching orders of the hosts of heaven that they take special note of the people who dwell in God. It is not to be wondered at that the servants are bidden to be careful of the comfort of their Master's guests; and we may be quite sure that when they are specially charged by the Lord himself they will carefully discharge the duty imposed upon them."

"The protection here promised is exceeding broad as to place, for it refers to all our ways, and what do we wish for more? How angels thus keep us we cannot tell. Whether they repel demons, counteract spiritual plots, or even ward off the subtler physical forces of disease, we do not know. Perhaps we shall one day stand amazed at the multiplied services which the unseen bands have rendered to us."

"as nurses carry little children, with careful love, so shall those glorious spirits upbear each individual believer ... even minor ills they ward off. It is most desirable that we should not stumble, but as the way is rough, it is most gracious on the Lord's part to send his servants to bear us up above the loose pebbles. If we cannot have the way smoothed it answers every purpose if we have angels to bear us up in their hands. Since the greatest ills may arise out of little accidents, it shews the wisdom of the Lord that from the smaller evils we are protected."

Angels are humbly slow to assert their own authority

Jude 1:9 But even the archangel Michael, when he was disputing with the devil about the body of Moses, did not himself dare to condemn him for slander but said, "The Lord rebuke you!" 

2 Peter 2:11  yet even angels, although they are stronger and more powerful, do not heap abuse on such beings when bringing judgment on them from the Lord. 

The allusion is an obscure one but the point is clear. Even Michael the Archangel, the head of all angels, did not take it upon himself to rebuke Satan or condemn his slander but meekly said "The Lord rebuke you!" . All the angels show the same modesty. In many cases they are stronger and more powerful then the fallen angels who oppose them but they do not heap abuse on such beings even when bringing judgment on them from the Lord. 

Angels praise God

Psalm 103:20-21 Praise the LORD, you his angels, you mighty ones who do his bidding, who obey his word. Praise the LORD, all his heavenly hosts, you his servants who do his will.

(See also Psalm 148:2 Praise him, all his angels; praise him, all his heavenly hosts.)

Angels have good reason to praise God - For their creation by God, for their preservation, for their happiness, in which they continue as they are elect of God in Christ.

They are always employed in the work of praising the Lord nor are they in the least backward in it, nor remiss. That is not why they are addressed as they are. Rather the design of the psalmist is only to show how great and good the Lord is and that angels, as great as they are, are under an obligation to praise him, and are bound to do so. He also intends to stir himself and others to this work, from such a consideration - that such noble creatures praise God.

C H Spurgeon

Finding his work of praise growing upon his hands, he calls upon "the firstborn sons of light" to speak the praises of the Lord, as well they may, for as Milton says, they best can tell. Dwelling nearer to that prepared throne than we as yet have leave to climb, they see in nearer vision the glory which we would adore. To them is given an exceeding might of intellect, and voice, and force which they delight to use in sacred services for him; let them now turn all their strength into that solemn song which we would send up to the third heaven. To him who gave angelic strength let all angelic strength be given. They are his angels, and therefore they are not loath to ring out his praises.

Angels will be judged one day - by believers

Paul says

1 Cor 6:3 Do you not know that we will judge angels? How much more the things of this life!

There is some mystery here but at the very least we can say that some angels will be judged and that believers will have some part in that judgement. No doubt that part will be a confirmatory one rather than anything more reliant on our own decisions. The fallen angels will be condemned to hell and the good angels will be confirmed for heaven. No doubt the focus lies on the fallen angels.

Angels revealed and announced the resurrection of Christ

Matthew 28:2-7 There was a violent earthquake, for an angel of the Lord came down from heaven and, going to the tomb, rolled back the stone and sat on it. His appearance was like lightning, and his clothes were white as snow. The guards were so afraid of him that they shook and became like dead men. The angel said to the women, "Do not be afraid, for I know that you are looking for Jesus, who was crucified. He is not here; he has risen, just as he said. Come and see the place where he lay. Then go quickly and tell his disciples: 'He has risen from the dead and is going ahead of you into Galilee. There you will see him.' Now I have told you."

Luke 24:2-7 They found the stone rolled away from the tomb, but when they entered, they did not find the body of the Lord Jesus. While they were wondering about this, suddenly two men in clothes that gleamed like lightning stood beside them. In their fright the women bowed down with their faces to the ground, but the men said to them, "Why do you look for the living among the dead? He is not here; he has risen! Remember how he told you, while he was still with you in Galilee: 'The Son of Man must be delivered over to the hands of sinners, be crucified and on the third day be raised again.'"

Angels - their negative work

1. Striking the Sodomites with blindness Genesis 19:11 Then they struck the men who were at the door of the house, young and old, with blindness so that they could not find the door.

2. Striking dead rebels in the wilderness in Moses' time 1 Corinthians 10:10 And do not grumble, as some of them did - and were killed by the destroying angel.

3. Willingness to slay Balaam by sword Numbers 22:31 Then the LORD opened Balaam's eyes, and he saw the angel of the LORD standing in the road with his sword drawn. So he bowed low and fell facedown. 

4. Driving out the Canaanites Exodus 33:2 I will send an angel before you and drive out the Canaanites, Amorites, Hittites, Perizzites, Hivites and Jebusites. 

5. Beginning to destroy the Israelites in David's time 1 Chronicles 21:15 And God sent an angel to destroy Jerusalem. But as the angel was doing so, the LORD saw it and relented concerning the disaster and said to the angel who was destroying the people, "Enough! Withdraw your hand." The angel of the LORD was then standing at the threshing floor of Araunah the Jebusite. (See 2 Samuel 24:16)

6. Slaughtering the Assyrians 2 Kings 19:35 That night the angel of the LORD went out and put to death a hundred and eighty-five thousand in the Assyrian camp. When the people got up the next morning - there were all the dead bodies! (See 2 Chronicles 32:21, Isaiah 37:36)

7. Striking Zechariah mute Luke 1:19, 20 The angel said to him, "I am Gabriel. I stand in the presence of God, and I have been sent to speak to you and to tell you this good news. And now you will be silent and not able to speak until the day this happens, because you did not believe my words, which will come true at their appointed time." 

8. Striking Herod dead Acts 12:23 Immediately, because Herod did not give praise to God, an angel of the Lord struck him down, and he was eaten by worms and died.

9. More generally Psalm 35:5, 6 May they be like chaff before the wind, with the angel of the LORD driving them away; may their path be dark and slippery, with the angel of the LORD pursuing them.

10. Various judgements Revelation 8;5 etc Then the angel took the censer, filled it with fire from the altar, and hurled it on the earth; and there came peals of thunder, rumblings, flashes of lightning and an earthquake. (See also Revelation 16, etc).