romans 11:25 For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in. - Free Bible Online

      25. For I would not . . . that ye should be ignorant of this mystery--The word "mystery," so often used by our apostle, does not mean (as with us) something incomprehensible, but "something before kept secret, either wholly or for the most part, and now only fully disclosed" (compare Ro 16:25; 1Co 2:7-10; Eph 1:9, 10; 3:3-6, 9, 10).
      lest ye should be wise in your own conceits--as if ye alone were in all time coming to be the family of God.
      that blindness--"hardness"
      in part is happened to--"hath come upon"
      Israel--that is, hath come partially, or upon a portion of Israel.
      until the fulness of the Gentiles be--"have"
      come in--that is, not the general conversion of the world to Christ, as many take it; for this would seem to contradict the latter part of this chapter, and throw the national recovery of Israel too far into the future: besides, in Ro 11:15, the apostle seems to speak of the receiving of Israel, not as following, but as contributing largely to bring about the general conversion of the world--but, "until the Gentiles have had their full time of the visible Church all to themselves while the Jews are out, which the Jews had till the Gentiles were brought in." (See Lu 21:24).

JFB.