The passage clearly says that they were proclaiming the gospel.
"Do not be afraid. I bring you good news (gospel) of great joy that will be for all people." (Lk. 2.10)
The gospel? What makes what the angels are doing the gospel. The answer is found not in what they are saying but in what they are doing.
The first people to hear of the birth of Jesus are social outcast. Shepherds were unable to keep up with all of the ceremonial requirements, such as the meticulous hand washings, so they were ostracized. Even though these shepherds could be guarding the lambs used in the sacrifices at the temple, they themselves do not sense that they are included in Gods plan. That is, until the angels show up and say…
"This will be a sign to you: You will find a baby wrapped in cloths and lying in a manger." v12
What made that a sign to THEM? What a shock it must have been to them that the true Lamb of God was lying in the midst of the smells of animal urine and feces. What good news that must have been!
Be an angel! Proclaim the good news to the margins so that it can be carried to the mainstream (v17)
Be an angel! Learn to proclaim the good news with "signs" that are truly good news to all who hear.
Be an angel with a jazzitude!