What does it do to a person to add works to faith for salvation?
Adding works to faith for salvation brings down the anathema of God Himself. The context explains that false teachers in Galatia claimed believers had to be circumcised and keep the Law of Moses to be saved, but such works do nothing except incur God's curse.
Les explains that Paul was frantically responding to false teaching in the Galatian congregations in Asia Minor, which is modern Turkey. False teachers insisted that salvation required circumcision and keeping the Law of Moses in addition to faith. Les notes that while circumcision is not practiced today, many add other works in the same category—things you can do—and this does nothing but bring down the anathema of God Himself.