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One technique or method I have found during my apologetics is to answer questions about theology with biblical passages only, in order to demonstrate how intensely “biblical” Catholic teaching is.
Regarding this particular issue, Catholics are often accused of “inventing” both confession and absolution (ie aspects of the sacrament of penance). In fact, no part of Catholic belief in this area is without explicit biblical sanction or model. Just do a little Bible study and “look around” a bit to find the information.
I will be using the RSV version.
1. The Church dispenses forgiveness (sacrament of penance)
Was the forgiveness of sins by priests foretold in the old covenant?
Leviticus 5: 5-6 When a man is guilty of any of these acts, he will confess the sin he has committed,  and he shall bring his trespass offering unto Jehovah for his sin which he hath committed, a female of the flock, or a lamb, or a goat, for a sin offering; and the priest shall make atonement for him for his sin.
Leviticus 19: 21-22 but he shall bring a sacrifice for himself unto the LORD at the door of the Tent of Meeting, a ram for a trespass offering.  And the priest shall make atonement for him with the ram for the trespass offering before Jehovah for his sin which he hath committed; and the sin he committed will be forgiven him.
Did the prophets ever function as God’s intermediaries in the area of forgiveness?
2 Samuel 12:13 David said to Nathan, “I have sinned against the Lord. “And Nathan said to David,” The Lord has also taken away your sin; you will not die.
Is there a passage showing that a priest grants absolution?
Matthew 18: 18b … Whatever you loose on earth will be loosed in heaven.
Does a priest have the authority to forgive sins as a representative of God?
John 20: 23a If you forgive someone’s sins, they are forgiven;… (cf. 2 Cor 2:10; see below)
Is Confession a Biblical Concept?
Psalm 38:18 I confess my iniquity, I regret my sin.
Proverbs 28:13 He who hides his transgressions will not prosper, but he who confesses them and forsakes them will obtain mercy.
Acts 19:18 Many also of those who were now believers came, confessing and disclosing their practices.
Is confession of sins to a spiritual authority specifically mentioned?
Mark 1: 4-5 John the Baptizer appeared in the wilderness preaching a baptism of repentance for the forgiveness of sins.  And he went out to him all the land of Judaea, and all the people of Jerusalem; and they were baptized by him in the river Jordan, confessing their sins. (cf. Mt 3, 6)
James 5: 14-16 Are any of you sick? Let him call the elders of the church, and let them pray over him, anointing him with oil in the name of the Lord;  and the prayer of faith will save the sick, and the Lord will raise them up; and if he has committed sins, he will be forgiven.  Therefore confess your sins to one another and pray for one another, that you may be healed. The prayer of a righteous man has great power in its effects. (cf. 1 Jn 1, 9)
2. Power of priests to impose penance
Do priests have the power to award penance to sinners?
Matthew 18: 18a Truly I say to you, whatever you bind on earth will be bound in heaven,. . .
Do priests have the power to “hold back” sins?
John 20: 23b … if you retirea in the sins of anyone they are retained.
Is penance after confession of sins seen in the old covenant?
Numbers 5: 7 he will confess his sin which he has committed; and he will fully restore his wrong, adding a fifth, and giving it to him to whom he has wronged.
Did Saint Paul assign penance to an unrepentant sinner?
1 Corinthians 5: 1-5 It is said, in fact, that there is immorality among you, and of a kind which is not even found among the heathen; for a man lives with his father’s wife.  And you are arrogant! Shouldn’t you cry instead? Let him who did this be removed from among you.  Because although absent from body, I am present of spirit, and as present, I have already pronounced the judgment  in the name of the Lord Jesus on the man who did such a thing. When you are assembled and my spirit is present, with the power of our Lord Jesus,  you must deliver this man to Satan for the destruction of the flesh, so that his spirit may be saved in the day of the Lord Jesus.
Did Paul even publicly name sinners to whom he assigned penance?
1 Timothy 1: 19-20 faith and a good conscience. By rejecting conscience, some people have wrecked their faith,  among them Hyménée and Alexandre, whom I delivered to Satan so that they learn not to blaspheme.
3. Indulgences (relaxation of temporal punishment)
Does the Church have the power to grant an indulgence?
2 Corinthians 2: 6-11 For such, this punishment of the majority suffices;  then you should instead turn to forgiving and comforting him, or he might be overwhelmed with excessive sadness.  So please reaffirm your love for him.  Because that’s why I wrote, so that I can test you and find out if you are obedient in everything.  Who you forgive, I forgive too. What I have forgiven, if I have forgiven anything, it is because of you in the presence of Christ,  to prevent Satan from gaining an advantage over us; for we are not ignorant of his designs.
Next time, then, you are asked “why do you confess to a man?” And so on, make sure you have this article handy!