"the world was created only for the sake of Israel"

A male convert to Rabbinic Judaism isn't even called a Jew, they're a "ger" (plural germin)

Germin is one of the Smaller Tractates of the Soferim, 8th century AD rabbinic literature, compiled after

the Talmuds, but considered to be an authority. The following is taken from the first English translation

of the Germin by Dr. Higger in 1930, regarding what a rabbi must say during the conversion process: 

After the immersion, they speak to him words of welcome, words of comfort: "Whom have you joined, O happy one!? You have joined Him Who created the world by mere uttering of words, blessed be He. For, the world was created only for the sake of Israel. There are none called the children of God, except Israel. There are none beloved of God, except Israel. All that we have spoken to you before your conversion, was only to increase your reward."

Seven Minor Treatises

Sefer Torah; Mezuzah; Tefillin; Zizit; Abadim; Kutim; Gerim.

And Treatise Soferim II.

Edited from mauscripts with an introduction, notes, variants and translation. 

By Michael Higger, Ph.D.

New York; Block Publishing Company. 1930

here for enlarged version

original found here