The expression "Finished Work Of Christ" means that there is nothing to be added to what has been accomplished by the Messiah on the cross of Calvary. Including a perfect justification of everyone, He has born the sins of. Hence, a person is not 'justified' by their act of believing on Christ, for that would mean Christ's work was not really finished at Golgotha. But a person is evidenced as justified by the fact of their believing in Christ Who died for them.