Tertullian,
Ante-Nicene Fathers Vol. III, Anti-Marcion…
(1885)
“ Should a dying man commit a deliberate sin and then expire, even after a life of love and obedience, who could fail to recognize the fearful nature of such an end? But, should his last word be one of infirmity and weakness, censurable but not involving wilful disobedience, surely we may consider it as provided for by the comfortable words—“there is a sin not unto death.” Yet “all unrighteousness is sin,” and the Fathers held that all sin should be repented of and confessed before God; because all sin when it is finished bringeth forth death.” ”
