Confessing our sins to a priest is very biblical:

“For every high priest chosen from among men is appointed to act on behalf of men in relation to God, to offer
gifts and sacrifices for sins.” (Heb 5:1)

“When a man is guilty in any of these, he shall confess the sin he has committed,  and he shall bring his guilt
offering to the LORD for the sin which he has committed, a female from the flock, a lamb or a goat, for a sin
offering; and the priest shall make atonement for him for his sin. ”  (Lev 5:5-6 )

"And the priest shall make an atonement for all the congregation of the children of Israel, and it shall be forgiven
them..." (Num 15:25)

When Jesus saw their faith, he said to the paralytic, "Child, your sins are forgiven." (Mk 2:5)

"For which is easier to say, "Your sins are forgiven you, or to say arise and walk? But that you may know that
the Son of Man has power to forgive sins" - then He said to the paralytic, "Arise, take up your pallet and go to
your house". And he arose, and went away to his house. But when the crowds saw it, they were struck with fear,
and glorified God who had given such power to men” (Mt 9:5-8)

"But that you may know that the Son of Man has authority to forgive sins on earth"-- he said to the paralytic, "I
say to you, rise, pick up your mat, and go home. He rose, picked up his mat at once, and went away in the sight
of everyone.” (Mk 2:10-12)  
Jesus has the authority to forgive sins!

Jesus gave this same authority to Peter.
I will give you the keys to the kingdom of heaven. Whatever you bind on earth shall be bound in heaven; and
whatever you loose on earth shall be loosed in heaven. (Mt 16:19)

Jesus later gave the same authority to all the Apostles.
“Amen, I say to you, whatever you bind on earth shall be bound in heaven, and whatever you loose on earth
shall be loosed in heaven.” (Mt 18:18)

“Therefore, if any one is in Christ, he is a new creation; the old has passed away, behold, the new has come. All
this is from God, who through Christ reconciled us to himself and gave us the
ministry of reconciliation; that
is, in Christ God was reconciling the world to himself, not counting their trespasses against them, and entrusting
to us the message of reconciliation. So we are ambassadors for Christ, God making his appeal through us. We
beseech you on behalf of Christ, be reconciled to God. For our sake he made him to be sin who knew no sin, so
that in him we might become the righteousness of God”  (2 Cor 5:17-21)

The Apostle John, an eyewitness, recorded more directly the words of Jesus giving the power to
forgive sins to all the Apostles after the testimony of the Resurrection.
"On the evening of that day, the first day of the week, the doors being shut where the disciples were, for fear of
the Jews, Jesus came and stood among them and said to them, "Peace be with you." When he had said this, he
showed them his hands and his side. Then the disciples were glad when they saw the Lord. Jesus said to them
again, "Peace be with you. As the Father has sent me, even so I send you." And when he had said this, he
breathed on them, and said to them, "Receive the Holy Spirit. If you forgive the sins of any, they are forgiven; if
you retain the  sins of any, they are retained." (Jn 20:19-23)

Therefore, confess your sins to one another and pray for one another, that you may be healed.” (James  5:16)  
Catholic's do this in Mass during the Penitential Rite.

“If we acknowledge (confess) our sins, he is faithful and just and will forgive our sins and cleanse us from every
wrongdoing.” (1 John 1:9)

“Peter (said) to them, "Repent and be baptized, every one of you, in the name of Jesus Christ for the
forgiveness of your sins; and you will receive the gift of the holy Spirit." (Acts 2:38)            

“If we acknowledge our sins, he is faithful and just and will forgive our sins and cleanse us from every
wrongdoing.” (1 John 1:9)
     
"I am writing to you, children, because your sins have been forgiven for his name's  sake.” (1 John 2:12)

“If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, let him be to
you as a Gentile and a tax collector.” (Matt 18:17)

Mortal & Venial Sins in the Bible:
If anyone sees his brother sinning, if the sin is not deadly (venial), he should pray to God and he will give him
life. This is only for those whose sin is not deadly. There is such a thing as
deadly sin (mortal), about which I
do not say that you should pray. All wrongdoing is sin, but there is sin that is not deadly. (1 Jn 5:16-17)        

List of various sins from the Bible:
(Jesus said) "But what comes out of a person, that is what defiles. From within people, from their hearts, come
evil thoughts, unchastity, theft, murder, adultery, greed, malice, deceit, licentiousness, envy, blasphemy,
arrogance, folly. All these evils come from within and they defile."  (Mk 7:20-23)

Now the works of the flesh are obvious: immorality, impurity, licentiousness, idolatry, sorcery, hatreds, rivalry,
jealousy, outbursts of fury, acts of selfishness, dissensions, factions, occasions of envy, drinking bouts, orgies,
and the like. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of
God. (Gal 5:19-21)

“... putting away falsehood ... Be angry but do not sin ... The thief must no longer steal ... No foul language
should come out of your mouths ... All bitterness, fury, anger, shouting, and reviling must be removed from you,
along with all malice.”  (Eph 4:25-31)

“Immorality or any impurity or greed must not even be mentioned among you, as is fitting among holy ones, no
obscenity or silly or suggestive talk, which is out of place, but instead, thanksgiving. Be sure of this, that no
immoral or impure or greedy person, that is, an idolater, has any inheritance in the kingdom of Christ and of
God. Let no one deceive you with empty arguments, for because of these things the wrath of God is coming
upon the disobedient. So do not be associated with them.”  (Eph 5:3-7)

“And do not get drunk on wine, in which lies debauchery” (Eph 5:18)

“But I say to you, everyone who looks at a woman with lust has already committed adultery with her in his heart.”
(Mt 5:28)

“But I say to you, whoever divorces his wife (unless the marriage is unlawful) causes her to commit adultery, and
whoever marries a divorced woman commits adultery.” (Mt 5:32)

“But I say to you, do not swear at all”  (Mt 5:34)

“But as for cowards, the unfaithful, the depraved, murderers, the unchaste, sorcerers, idol-worshipers, and
deceivers of every sort, their lot is in the burning pool of fire and sulfur, which is the second death.” (Rev 21: 8)

“Do you not know that the unjust will not inherit the kingdom of God? Do not be deceived; neither fornicators nor
idolaters nor adulterers nor boy prostitutes nor practicing homosexuals nor thieves nor the greedy nor drunkards
nor slanderers nor robbers will inherit the kingdom of God.” (1 Cor 6:9-10
)
Confession & Sins
Catholics believe, as do some other Non-Catholic Christians, that Jesus gave
to the Apostles and their successors the power to forgive sins, reconciling
sinners to God for sins committed after Baptism.
The priest says in the confessional, "I absolve you of your sins." The priest is acting in Personna
Christi
, or in the person of Christ. Since the priest is acting in the person of Christ, then it is Christ to
whom you confess your sins. It is Christ alone who remits them. Jesus Christ uses the priest as His
voice and His hands.  The prophets of the Old Testament spoke in GOD's name. They spoke in the
person of God. The priests of the New Covenant speak in the person of Christ.  God never changes.