A compound verb from epi (upon, in addition) + martureō (testify), meaning to add testimony, confirm by witness, or bear additional attestation. Used by Peter to confirm that the grace of God is genuine in the believers to whom he writes.
Peter's farewell assurance in 1 Peter 5:12 — 'I have written briefly to you, testifying that this is the true grace of God' — uses epimartureō as his final seal. After five chapters addressing suffering, submission, and the hope of glory, he frames the entire letter as added testimony. The word belongs to the judicial sphere — a secondary witness who confirms what the primary witness has already established. For Peter, the primary witness is Christ Himself; Peter's letter is the confirming voice. All faithful preaching and teaching stands in this position: we are not the originating word but the confirming echo — witnesses to the Witness, testifiers to the True.