About 20 minutes into Handel's 'Messiah there is a chorus about the Messiah being like a refiner's fire.... "and He shall purify... the sons of Levi". This is taken from Malachi 3:3. My question is, why is it only the Levites that are singled out for purification? What about all the other tribes of Israel?
The significance of the sons of Levi was that they were the priestly tribe of Israel, and had special duties in the temple, offering sacrifices and leading worship. The priests had to be purified in order to perform their duties, as is referred to in the verses surrounding v. 3. In performing their duties, the sons of Levi facilitated the purification of the other tribes. It isn't that the Sons of Levi were the only ones singled out for purification, but that they were the first ones singled out for purification. One analogue to this might be in the safety instructions about flying, when the passenger traveling with small children is instructed, in the case of emergency, to put on their oxygen mask, and then to put on the mask(s) of the children in their care. In like manner, the Sons of Levi were purified first, so they would facilitate the purification of the rest of Israel.