When the seventy translated the Tanakh into Greek, they had to find a word for berit, and they had two candidates. Syntheke was the ordinary Greek for an agreement, a treaty, a contract struck between parties who meet as equals and negotiate. Diatheke was something else: a disposition, an arrangement laid down by one party who has the standing to set the terms, which is why it was also the ordinary word for a last will.

They chose diatheke, and they chose it nearly three hundred times. The reason is theological, not linguistic. Israel did not negotiate at Sinai. HaShem set the terms, and He set them after He had already brought them out. A berit is not a deal between equals and syntheke would have said it was.

Then English happened. Diatheke went into Latin as testamentum, and testamentum went into English as testament, and that is how the two halves of your Bible ended up named after a legal document you sign before you die instead of after the covenant relationship they actually describe. Old Testament and New Testament should read Old Covenant and New Covenant. It is not a small loss. A testament is a paper that takes effect when someone dies and is executed by professionals on behalf of beneficiaries who were not present. A covenant is a bond between living parties who each stand in it. Read the word as testament and you will look for an inheritance. Read it as diatheke, and behind it berit, and you will look for the One who set the terms and for the people He set them with.