Why Didn’t God Explain the Trinity Explicitly in Scripture?

Questions about whether the Old Testament believers were regenerate, how the work of the Spirit is different in the New Testament, and why God didn’t explain the Trinity explicitly in Scripture.

• Were Old Testament believers regenerate, and how is the work of the Spirit different in the New Testament?
• I agree the Trinity best explains all the biblical data, but why didn’t God lay it out explicitly in Scripture?