Galatians 3:15-25

v15-18: The way of justification by faith in God and His promises was established before God gave the law to Israel.  "It would be inconsistent with [God's] holiness, wisdom, and faithfulness" to alter the way someone is justified.  We can be sure that the way of salvation has not and will not change because God does not lie.  Abraham was justified by faith in the promised Christ and it only by faith in Christ that we will be.

v19-22: If the law does not save, why was it given to Israel?  "The law was given to convict them of their sin."  It was "also intended to restrain them from committing sin, to put reverence in their hearts and restrain their sinful desires."  At the same time "it was intended to direct them to the true and only way by which sin could be atoned for and in which they could obtain pardon, namely, through the death and sacrifice of Christ."

v23-25: The law was a means God gave to convince people "of their lost and ruined condition in themselves and to let them see the weakness and insufficiency of their own righteousness to commend them to God."  The way the law did this was by declaring "the mind and will of God about them," and at the same time declaring "a curse against them for every failure in their duty."