And, finally, it is testamentary as a free and sovereign disposition on the part of God. It is called a 'testament' in Heb. The covenant of grace differs from the covenant of works in that it has a mediator. He is Mediator, not only merely in the sense that He intervenes between God and man to sue for peace and to persuade to it, but in the sense that He is armed with full power to do all that is necessary for the actual establishment of peace. Membership in the covenant.
Adults can enter the covenant as a purely legal arrangement only by faith.
And when they so enter it, they at the same time gain entrance into the covenant as a communion of life. They therefore enter upon the full covenant life at once. Children of believers, however, enter the covenant as a legal arrangement by birth, but this does not necessarily mean that they also at once enter it as a communion of life, nor even that they will ever enter it in that sense. Yet the promise of God gives a reasonable assurance that the covenant life will be realized in them. As long as they do not manifest the contrary we may proceed on the assumption that they possess the new life.