Though Abraham kept all the commandments, he was not perfect till he was circumcised.
Nedarim, fol. 31, col. 2.
In whatever sense this may have been written, and whatever the interpretation that may be put upon it, there is one sense in which it is absolutely and eternally true, and that is, that, in order to be perfect, a man's life must be as pronounced on the negative side as the positive, in its denials as in its affirmations, and that it is futile to attempt to obey God unless one at the same time renounce all co-partnery with the devil. Circumcision is the symbol of this renunciation, and it is only as such it has any radical spiritual significance. Till he was circumcised, it is said, God did not speak to Abraham in Hebrew. Not till then is sacredness of speech, any more than sacredness of life, possible. Doubtless among the Jews circumcision was the symbol of their separation from the ethnic religions; and hence the jealousy with which their prophets looked upon any compromise with idolatry. Hatred of that, utter and intense, was the one essential negative pole of genuine Judaism, and circumcision was its sign and seal.
Abraham was the first of the proselytes.
Succah, fol. 49, col. 2.
Abraham it was that ordained the form of prayer for morning worship, which is extant to this very day.
Berachoth, fol. 26, col. 2.
As he himself was pious, so were his very camels, for they would not enter into a place where there were idols; as it is written (Gen. xxiv. 31), "I have prepared," i.e., removed the idols from, "the house and room for the camels."
Avoth d' Rabbi Nathan, chap. 8.