The words “Covenant” and “Testament” (Greek=DIATHEKE) are often used interchangeably in the Bible (Heb 9:4, 15-17) because a Testament is a document containing the authentic stipulations of a Covenant agreement, a pact, a pledge or a will.
Covenants are solemn agreements enacted by God as provision for His peculiar intimacy with humanity. Entrance into Divine redemptive Covenants is always entrance into responsible stewardship of God’s unmerited grace.
The New Testament Scriptures document the authenticated provisions of the New Covenant as attested by Christ through His life and teachings, and sealed in Christ’s Blood which He shed at His Death on the Cross (Mar 14:24; 1Cor 11:25; Heb 9:15-20; 13:20).
Christ instituted the New Covenant and sealed it with His death and blood as the Testator and Mediator (Heb 9:15-17), Surety or Guarantee (Heb 7:22), Author and Finisher (Heb 12:2),
Christ appointed the Apostles as His authenticated eye-witnesses (Luk 24:48; Act 1:22; 3:15) and anointed them as able ministers of His New Covenant? (2Cor 3:6).
The Gospels record the testamentary “word and deed” (Acts 1:1) depositions of Christ’s New Covenant which are elaborated upon in the books of Acts, Epistles and Revelation.
Old Testament Scriptures record the Old Covenant provisions mediated by Moses in the Torah which are elaborated upon in the Prophets and the Writings.
Those who believe in Christ become partakers of the Salvation He offers freely in His New Covenant.