An Excerpt From Dickson's Commentary on WCF Chapter VII: Of God’s Covenant With Man
Here Dickson (c. 1583-1663) argues that the Old and New Covenants are not two different covenants, but actually expressions of the one Covenant of Grace. The New Testament and Old Testament do not differ then in substance but only in accident (its manner or shape fitting to its time and use). The essential unity of the Old and New Covenants is seen clearly in that both contain the same spiritual blessings: the promise of grace, forgiveness, and eternal life and blessing for believers in Jesus Christ. In addition, both covenants contain the requirement of the same faith and obedience.
Are there two Covenants of Grace “differing in substance” or but one and the same “under various dispensations?”
One only: Galatians 3:14,16. Acts 15:11. Romans 3:21, 22, 23, 30. Romans 4:3, 6, 16, 17, 23, 24. Hebrews 13:8.
Well then do not the Socianians err, who maintain a substantial and not an accidental difference between the Old Covenant and the New?
By what reasons are they confuted?
First, because in both the covenants there is the same promise of grace, concerning remission of sin and life eternal, freely to be given to believers for Christ’s sake. Genesis 3:15, where the seed of woman is promised to bruise the head of the serpent. Genesis 17:7, where it is said “I will be thy God and the God of the seed after thee.” Genesis 22:18, where it is said “In thy seed shall all the nations of the earth be blessed.” And John 3:36, where it is said “he that believeth hath everlasting life.” And Acts 15:11, where it is said “But we believe that through the grace of our Lord Jesus Christ, we shall be saved even as they.”
Second, because one and the same faith and obedience, on both sides, is required. “Walk before me and be thou perfect” says Genesis 17:1, and in Mark 1:15, “Christ says after he came to Galilee preaching the gospel of the kingdom, ‘The time is fulfilled, and the kingdom of God is a hand; Repent ye, and believe the gospel.’”
“Truth’s Victory Over Error” by Church of Scotland minister Rev. David Dickson, is the earliest known commentary on the Westminster Confession. It is a translation by George Sinclair of Dickson's Latin lectures on the confession of faith, “Praelectiones in confessionem fidei”. this excerpt is found on p. 70-71 and can be viewed in context at https://archive.org/details/truthove00dick