When the bible says Christ bought us or redeemed us from the curse of the law, being made a curse for us, it’s talking about penal substitution. God’s law and justice demand the punishment of sinners, so we were lawbreakers and under a sentence of death. Christ redeems us by penal substitution – He satisfies justice by His death.
The same cannot be said for faith. The bible never says Christ bought or redeemed faith. Does faith need to be rescued? Is faith a lawbreaker and under a sentence of death? It’s not like the bible speaks of Christ’s death overcoming some obstacle to God’s giving us faith. Rather, scriptures speak of God redeeming us from sin’s penalty.
1 For a more recent example, see Schrock’s chapter 4 in Whomever He Wills, page 103.