Answer: Although Christ, by his obedience and death, did make a proper, real, and full satisfaction to God’s justice in the behalf of them that are justified₁; yet inasmuch as God accepts the satisfaction from a surety, which he might have demanded of them, and did provide this surety, his own only Son₂, imputing his righteousness to them₃, and requiring nothing of them for their justification but faith₄, which also is his gift₅, their justification is to them of free grace.₆

1. Romans 5:8-10; Romans 5:19
2. 1 Timothy 2:5-6; Hebrews 10:10; Matthew 20:28; Daniel 9:24; Daniel 9:26; Isaiah 53:4-6; Isaiah 53:10-12; Romans 8:32; 1 Peter 1:18-19
3. 2 Corinthians 5:21
4. Romans 3:24-25
5. Ephesians 2:8
6. Ephesians 1:7

For further study see:

1. WSC 32: What benefits do they that are effectually called partake of in this life?
2. WSC 33: What is justification?
3. WSC 36: What are the benefits which in this life do accompany or flow from justification, adoption, and sanctification?
4. WLC 70: What is justification?
5. WLC 77: Wherein do justification and sanctification differ?
6. WCF CHAPTER 11:  Of Justification