Luke 18:13 KJV
And the publican, standing afar off, would not lift up so much as his eyes unto heaven, but smote upon his breast, saying, God be merciful to me a sinner.
Scofield's Notes says," the Bible knows nothing of divine forgiveness apart from sacrifice."
And then describes how even this publican, who, being a Jew, would have been instructed in the Law, understood in that Temple that sacrifices are offered for the atoning of sins. He would not have been thinking of mere mercy, but of mercy springing from the blood sprinkled at God's altar.
So today we have people crying out to God for mercy but some of them are doing it without effect because they hope for mercy with no merit. Mercy is always undeserved, but it is never without merit. Mercy can only come through sacrifice and that sacrifice must be the blood of Jesus Christ.
Mercy comes through appealing to God through the blood of Christ.