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Herman Bavinck, Reformed Dogmatics Volume 3.
Pg 215-216
"...the Old Testament is also to be viewed as one in essence and substance wth the New Testament. For though God communicates his revelation successively and historically and makes it progressively richer and fuller, and humankind therefore advances in the knowledge, possession, and enjoyment of revelation, God is and remains the same. The sun only gradually illumines the earth, but itself remains the same, morning and evening, during the day and at night. Although Christ completed his work on earth only in the midst of history and although the Holy Spirit was not poured out till the day of Pentecost, God nevertheless was able, already in the days of the Old Testament, to full distribute the benefits to be acquired and applied by the Son and the Spirit. Old Testament believers were saved in no other way than we. There is one faith, one Mediator, one way of salvation, and one covenant of grace."
Page 221-222
"The benefits granted to Israel by God in this covenant (Sinai) are the same as those granted to Abraham, but more detailed and specialized. Genesis 3:15 already contains the entire covenant in a nutshell and all the benefits of grace. God breaks the covenant made by the first humans with Satan, puts enmity between them, brings the first humans over to his side, and promises them victory over the power of the enemy. The one great promise to Abraham is "I will be your God, and you and your descendants will be my people" *Gen 17:8 paraphrase). And this is the principle content of God's covenant with Israel as well. God is Israel's God, and Israel is his people (Exod 19:6; 29:46; etc.). Israel, accordingly, receives a wide assortment of blessings, not only temporal blessings, such as the land of Canaan, fruitfulness in marriage, a long life, prosperity, plus victory over its enemies, but also spiritual and eternal blessings, such as God's dwelling among them (Exod. 29:45; Lev. 26:12), the forgiveness of sins (Exod. 20:6, 34:7; Num. 14:18; Deut. 4:31; Pss. 32; 103; etc.), sonship (Exod. 4:22; 19:5-6, 20:2; Deut. 14:1; Isa 63:16; Amos 3:1-2; etc.), sanctification (Exod. 19:6, Lev. 11:44, 19:2), and so on. All these blessings, however, are not as plainly and clearly pictured in the Old Testament as in the New Testament. At that time they would not have been grasped and understood in their spiritual import. The natural is first, then the spiritual. All spiritual and eternal benefits are therefore clothed, in Israel, in sensory forms. The forgiveness of sins is bound to animal sacrifices. God's dwelling in Israel is symbolized in the temple built on Zion. Israel's sonship is primarily a theocratic one, and the expression "people of God" has not only a religious but also a national meaning. Sanctification in an ethical sense is symbolized in Levitical ceremonial purity. Eternal life, to the Israelite consciousness, is concealed in the form of a long life on earth. It would be foolish to think that the benefits of forgiveness and sanctification, of regeneration and eternal life, were therefore objectively nonexistent in the days of the Old Testament. They were definitely granted then as well by Christ, who is eternally the same....The spiritual an eternal clothed itself in the form of the natural and temporal. God himself, Elohim, Creator of heaven and earth, as Yahweh, the God of the covenant, came down to the level of the creature, entered into history, assumed human language, emotions, and forms, in order to communicate himself with all his spiritual blessings to humans and so to prepare for his incarnation, his permanent and eternal indwelling in humanity. We would not even have at our disposal words with which to name the spiritual had not the spiritual first revealed itself in the form of the natural.
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