Of course God has promised to make those that believe His Word a peculiar treasure unto Himself. He did not say He would make those who refused to believe and receive His word a peculiar people.Another scripture that speaks of the Church that applies to 1 Pet 2:9
9 But ye are a chosen generation, a royal priesthood, an holy nation, a peculiar people; that ye should shew forth the praises of him who hath called you out of darkness into his marvellous light;
Its Eph 1:14 when speaking of the Church comprised of both jew and gentile believers in Ephesus
Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.
Now what is the purchased possession ? Why its the Church, bought with a price, as here Acts 20:28
28 Take heed therefore unto yourselves, and to all the flock, over the which the Holy Ghost hath made you overseers, to feed the church of God, which he hath purchased with his own blood.
Now the words purchased possession in Eph 1:14 is the same word for " a peculiar people " in 1 Pet 2:9, now how so that ? Because they're the same exact people !
Now therefore, if ye will obey my voice indeed, and keep my covenant, then ye shall be a peculiar treasure unto me above all people: for all the earth is mine: