Hebrews 10:19-23: "19 Therefore, brothers, since we have confidence to enter the holy places by the blood of Jesus, 20 by the new and living way that he opened for us through the curtain, that is, through his flesh, 21 and since we have a great priest over the house of God, 22 let us draw near with a true heart in full assurance of faith, with our hearts sprinkled clean from an evil conscience and our bodies washed with pure water. 23 Let us hold fast the confession of our hope without wavering, for he who promised is faithful."
Psalm 51:1-2: "1 Have mercy on me, O God, according to your steadfast love; according to your abundant mercy blot out my transgressions. 2 Wash me thoroughly from my iniquity, and cleanse me from my sin!"
Hebrews 4:16: "Let us then with confidence draw near to the throne of grace, that we may receive mercy and find grace to help in time of need."
1 John 1:9: "If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness."
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Most of the cited passages mention cleansing, and any of these could be cited for the hymn's title line, which occurs at the beginning of the first and fifth verses.
The lines "As I approach on bended knee; / My Father's mercy here I plead" in the first verse seem to be taken from Hebrews 4:16 (specifically: "draw near to the throne of grace, that we may receive mercy"), although "draw near with a true heart" from Hebrews 10:22 could also be cited.
1 John 1:9 appears in the line "I lay before Him all my sin" in the second verse and in the general theme of cleansing.
The text from Hebrews 10 (particularly verses 19-20) also seems to appear in the lines "Lord, may Your wounded hand impart / Your healing to my broken heart" in the third verse and "Christ's wounded hand has set me free" in the fifth.
