The terms 'Old Testament' and 'New Testament' are so familiar in English that their origins as translated theological concepts are easily overlooked. Yet these two phrases shaped not only religious vocabulary but the language of law, contract, and binding agreement that underlies much of Western institutional life.
The underlying Greek word is diatheke, which in classical Greek means primarily a last will and testament - a legal disposition of property effective on the death of the maker. In the Septuagint (the Greek translation of the Hebrew scriptures), diatheke was used to translate the Hebrew berith (covenant, treaty, or binding agreement), slightly shifting the semantic register from bilateral agreement to unilateral disposition. Paul and the author of Hebrews developed this ambiguity theologically: Christ's death made his diatheke - his new covenant/testament - legally effective (Hebrews 9:15-17). The death of the testator activated the testament.
Tertullian (c. 160-220 CE), writing in Latin, used testamentum (the Latin for a legal will) to translate diatheke, and began using 'Old Testament' and 'New Testament' as terms for the two collections of scripture. The terminology stuck, spread through the Latin church, and entered all Western European languages when the Bible was translated into vernaculars. English received it through Wycliffe, Tyndale, and ultimately the King James Version.
The legal resonance of 'testament' shaped English legal vocabulary. In English law, 'last will and testament' became the standard formula for a legal disposition of one's estate at death - with 'will' and 'testament' functioning as near-synonyms covering slightly different aspects of the document (will covers real property; testament covers personal property in older usage). The phrase 'to bear testament' (to give testimony) connects the legal and religious senses. 'Testify' and 'testimony' derive from the same Latin root (testis, a witness), weaving together the legal and scriptural vocabularies at the root level.
The phrase 'New Testament' also entered legal English as a description of a revised or superseding will. If a person makes a will and then makes another, the second is sometimes described as a 'new testament' to the first - a usage that echoes the theological relationship between the two scriptural collections.
In broader cultural usage, 'Old Testament' and 'New Testament' function as shorthand descriptors for two contrasting styles or eras. An 'Old Testament' approach to something is severe, law-focused, retributive; a 'New Testament' approach is gracious, relational, forgiving. This contrast is often theologically naive (the Hebrew Bible contains extraordinary passages of grace; the New Testament contains severe warnings of judgment), but it functions as a persistent cultural shorthand. A 'fire-and-brimstone' preacher is 'very Old Testament'; a pastor known for pastoral warmth is 'very New Testament.'
The terms also entered comparative religion as neutral academic categories - scholars speak of 'Old Testament studies' and 'New Testament studies' as distinct academic disciplines, with their own professional societies, journals, and methodological traditions. The phrase 'Hebrew Bible' has been adopted in many academic contexts as a less Christian-normative alternative to 'Old Testament,' reflecting awareness that the designation 'Old' carries implicit theological judgment. Nonetheless, 'Old Testament' and 'New Testament' remain the dominant terms in popular usage worldwide.