Westminster Confession 7:2-3
2. The first covenant made with man was a covenant of works in which life was promised to Adam and, in him, to his posterity, upon condition of perfect and personal obedience.
3. Since man, by his fall, made himself incapable of life by that covenant, the Lord was then pleased to make a second covenant, commonly called the covenant of grace. In it God freely offers life and salvation by Jesus Christ to sinners, requiring of them faith in him, that they may be saved, and promising to give his Holy Spirit to all those who are ordained to eternal life, to make them willing and able to believe.
The Covenant of Life, most often called the Covenant of Works, is a gracious covenant in which God promises life to Adam and his descendants upon perfect obedience. Adam was able to sin or not to sin, so fulfilling the requirement of obedience for the probationary period was a real possibility. But Adam, the unfallen man, failed. His failure, and its consequences were passed on to his offspring.
Now, although the Covenant of Works is still in force, (we are all condemned by it) no one can achieve life through it. The Puritans called it the old, broken Covenant of Works. God decided to commit to another covenant, the Covenant of Grace. Perfect obedience is still required. Christ, by his active and passive obedience, (by his obeying of the Law of God perfectly and by his submitting to his penalty thoroughly), fulfilled the Covenant of Grace. I sometimes say that the Covenant of Grace was a Covenant of Works for Christ. He undid what the first Adam had done. He obeyed perfectly where the first Adam failed. We, due to Christ’s fulfilling the Covenant of Grace, freely receive life and salvation through faith in Jesus Christ. This covenant was made with Christ and through him all the elect. Salvation in its full sense is promised upon the condition of faith. We receive not only forgiveness, acceptance, and justification, but also the Holy Spirit and through Him the ability to persevere in the faith to the end.