by Herman Witsius
XXVIII. The Testament is the Will of God, or that “counsel of his will,” by which he has appointed both the inheritance and the heirs, and to which our Lord referred, when he said, “It is your Father’s good pleasure to give you the kingdom.” I add, that it is the last and irrevocable will of the Father; for as this is essential to a valid testament among men, so it is not wanting to this testament. “Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath, that by two immutable things, in which it was impossible for God to lie, we might have a strong consolation.” In this Will, he has assigned the inheritance as well of grace as of glory, of which we shall speak immediately. He has also appointed the heirs, – not indefinitely, all that shall believe; but these and the other persons particularly, whose “names are written in heaven,” and “graven upon the palms of God’s hands.” This his Will, he has expressed in both parts of the holy Scriptures, which are, therefore, called a Testament. In fine, that this Will might in no respect be defective, the whole is confirmed and sealed by the blood and death of the Lord Jesus.
XXIX. To understand this, we must observe, that God the Father, did, by testament, entrust his Son Jesus with this honour, that he should be the head of the elect, to excel them in glory, and to possess authority to impart to them, all his blessings. Jesus, again, by the power committed to him by the Father, bequeathes his benefits by testament, to the elect, that they may be joint-partakers of them with himself. “I appoint to you (by testament) a kingdom, as my Father hath (by testament) appointed unto me.” This making of the Testament, then, is originally the doing of the Father, but immediately of Christ the Mediator; who died, not to make void the inheritance by his death, for he is “alive for evermore,” but to seal the promises, and to acquire for his people a right to the inheritance. Hence the blood which he shed, is called “the blood of the testament.”
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From Witsius' Sacred Dissertations, 1:168ff.: