An adaptation of the Catechism for Young Children in modern English.
Question: What did Christ agree to do in the covenant of grace?
Answer: To keep the whole law for His people, and to make atonement for their sins.
Question: What did Christ undertake in the covenant of grace?
Answer: To keep the whole law for his people, and to suffer the punishment due to their sins.
“Undertake” is simplied to “agree to do”. “Agree” also has the added benefit of matching the definition given for covenant in question 22. Additionally, several questions later focuses on the particular category of penal subsitutionary atonement, so we adopt the technical language “make atonement” and allow the future question to clarify the meaning. C.f. questions 28, 29 and 30.