Used in Christian contexts to refer to a person who testifies to their faith, often a martyr or someone who bears witness to Christ.
He dedicated his life as a witness for Christ.
Compound of 証し (akashi, 'testimony, proof') + 人 (hito, 'person'), with rendaku voicing hito → bito. The spelling 証し人 reflects the Christian usage of 'witness' as one who testifies to their faith.