Midrasch zu Malachi 3:3

Rabba b. Huna said: "There is one great preference between an Israelite and a Proselyte. Concerning the Israelite the passage reads (Ez. 37, 27) And I will be their God, and they shall be My people, but concerning a Proselyte it is written (Jer. 30, 21) For who is this that has pledged his heart to approach unto Me?" R.Chelbo said:"Proselytes are as bad to Israel as a sore on the skin; for it is written (Is. 14, 1) And the stranger shall be joined unto them, and they shall attach (V'niss'pechu) themselves to the house of Jacob. It is written here [in the Text] V'niss'pchu, and it is written [concerning leprosy] the same expression (Lev. 14, 56) And for a swelling and for a rising (Sapachath)." R. Chama b. Chanina said: "At the time when the Holy One, praised be He, (Fol. 71) will purify the tribes He will commence with the tribe of Levi first, as it is said (Mal. 3, 3) And He will sit as a melter and purifier of silver; and He will purify the sons of Levi, and refine them as gold and silver, that they may offer unto the Lord an offering in righteousness." R. Joshua b. Levi said: "Impure families who joined Israel through the influence of their wealth, will be purified, as it is said (Mal. 3, 3) And He will sit as a melter and purifier of silver. What does the passage (Ib.) That they may offer unto the Lord an offering in righteousness, mean? Said R. Isaac: "Righteousness has the Lord done with Israel that every family which was mixed up beyond recognition is considered pure."

An idolater asked Rabban Gamliel, "Why was the Holy One, Blessed be He revealed to Moses in the Burning Bush?" Rabban Gamliel replied to him, "If God had been revealed in a carob tree or a fig tree, you would have asked me the same thing, and I could not send you away without an answer. This teaches you that there is no place in the world devoid of the Shekhinah."