“We cannot by our best works merit pardon of sin” (XVI. 5).
The WCF says that they cannot by their best works merit pardon for sin, but concluding from what they say in XVIII. 3-4, they believe that their assurance of pardon for sin DOES come by their best works.
“Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin” (Romans 3:20).
In view of WCF XVIII.3-4, they would say that while no one is justified by the deeds of the law (i.e., one’s own efforts) in God’s sight; nevertheless one can obtain assurance that one is justified in His sight by one’s own efforts.
“But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness” (Romans 4:5).
Works cannot form even the minutest ground on which assurance of justification is based. Works cannot play any part in assurance or certainty of justification since justification is to him that worketh not, but believeth. Justification AND assurance of justification are both grounded upon the atoning blood and imputed righteousness of Christ alone.
Agrammatos 