CLAIM: Critics argue that Peter is twisting this OT passage to make it appear to be prophetic of Christ. In the Masoretic Text (MT), the Hebrew lacks the expression “in him.” Therefore, when Peter writes, “And he who believes in Him will not be disappointed” (1 Pet. 2:6), he is adding onto the original Hebrew text. Is this the case?
RESPONSE: Not every citation of the OT is a verbatim quotation. Sometimes, the NT authors are merely intending to cite the OT loosely. For instance, in John 19:37, John writes, “They shall look on Him whom they pierced.” Of course, here John is citing Zechariah 12:10, which states, “They will look on Me whom they have pierced.” Did John misquote this passage? Of course not. John was simply altering this passage to refer to Jesus, as a third person observer. If Jesus had written the passage, he would have cited it from the first person perspective (“They will look on Me…”).
In this passage, Peter is connecting Isaiah 28 with a messianic fulfillment. Isaiah 28:16 merely commands the people to believe, but Peter is filling in the gap for them to believe in Christ (i.e. the ultimate object of their belief). When we read the context of Isaiah 28, we see that this “cornerstone” will bring justice and righteous over and against the Pagan nations (v.15, 17), which is a clear messianic theme. For this reason, other ancient Jewish interpreters also understood this passage to be messianic. For instance, the Targum of Isaiah 28:16 and Psalm 118:22ff both interpret the “cornerstone” to be king messiah.
 Clowney, E. P. The message of 1 Peter: The way of the cross. The Bible Speaks Today. Leicester, England; Downers Grove, IL: InterVarsity Press. 1988. 84.