What's the biblical response to Jehovah's Witnesses denying the Trinity?

It's true the word "Trinity" never appears in Scripture, but the concept it summarizes clearly does: one God eternally existing in three persons — Father, Son, and Holy Spirit.

The Father is called God (1 Peter 1:2), the Son is called God (John 1:1, Titus 2:13), and the Holy Spirit is called God (Acts 5:3-4), yet Scripture insists there is only one God (Deuteronomy 6:4, Isaiah 45:5). At Jesus's baptism, all three persons are present distinctly in the same moment (Matthew 3:16-17) — the Father speaking, the Spirit descending, the Son being baptized.

The Trinity isn't a later invention; it's the church's attempt to faithfully hold together everything Scripture says about the Father, Son, and Spirit without denying any part of it.