Westminster Confession of Faith, Chapter VII, Section IV–This covenant of grace is frequently set  forth in Scripture by the name of a testament, in reference to the death of Jesus Christ the testator, and to the everlasting inheritance, with all things belonging to it, therein bequeathed.

A. A. Hodge expounds:

Christ as mediatorial King, administers to his people the benefits of his covenant; and by his providence, his Word, and his Spirit, he causes them to become severally recipients of these blessings, according to his will. These benefits he offers to all men in the gospel. He promises to grant them on the condition they are received. In the case of his own people, he works faith in them, and as their Surety engages for them and makes good all that is suspended upon or conveyed through their agency. In the whole sphere of our experience every Christian duty is a Christian grace; for we can fulfill the conditions of repentance and faith only as it is given to us by our Surety. All Christian graces also involve Christian duties. So that Christ at once purchases salvation for us, and applies salvation to us; commands us to do, and works in us to obey; offers us grace and eternal life on conditions, and gives us the conditions and the grace and the eternal life. What he gives us he expects us to exercise. What he demands of us he at once gives us. Viewed on God’s side, faith and repentance are gifts of the Son. Viewed on our side, they are duties and gracious experiences, the first symptoms of salvation begun–instruments wherewith further grace may be attained. Viewed in connection with the covenant of grace, they are elements of the promise of the Father to the Son, conditioned upon his mediatorial work. Viewed in relation to salvation, they are indices of its commencement and conditions sine qua non [something that is essential, something that is indispensable] of its completion (A. A. Hodge, The Confession of Faith, 127-128).