It is represented many, many times in God's cry to his people, but it specifically shows up regarding the coming new covenant in Isaiah.
"The voice of one crying in the wilderness, 'Prepare the way of the Lord, make his paths straight, every valley shall be filled, and every mountain and hill shall be made low, and the crooked shall become straight, and the rough places shall become level ways, and all flesh shall see the salvation of God' ". Isaiah 40:3-5
It shows up immediately in the New Testament as God introduces his new covenant. Zechariah, the father of John the Baptist, pronounces these words over his infant son as he was "....filled with the Holy Spirit and prophesied, saying..."
"...And you, child, will be called the prophet of the Most High; for you will go before the Lord to prepare his ways, to give knowledge of salvation to his people in the forgiveness of their sins, because of the tender mercy of our God, whereby the sunrise shall visit us from on high to give light to those who sit in darkness and in the shadow of death, to guide our feet into the way of peace."
When this child became a man and made his public appearance to Israel, this was his message,
"And he went into all the region around Jordan, proclaiming a baptism of repentance for the forgiveness of sins." Luke 3:3
When large crowds appeared, he did not exactly seek to make them feel warm and cozy.
"He said therefore to the crowds that came out to be baptized by him, 'You brood of vipers! Who warned you to flee from the wrath to come? Bear fruits in keeping with repentance. And do not begin to say to yourselves, We have Abraham as our father. For I tell you, God is able from these stones to raise up children for Abraham. Even now the axe is laid to the root of the trees. Every tree therefore that does not bear good fruit is cut down and thrown into the fire." Luke 3:7-9
But...he did not leave them with a curse, but WHEN THE PEOPLE ASKED, he gave them hope and instruction...
"And the crowds asked him, 'What then shall we do?' And he answered them, 'Whoever has two tunics is to share with him who has none, and whoever has food is to do likewise.' Tax collectors also came to be baptized and said to him, 'Teacher, what shall we do?' And he said to them, 'Collect no more than you are authorized to do.' Soldiers also asked him, 'And we, what shall we do?' And he said to them, 'Do not extort money from anyone by threats or by false accusation, and be content with your wages.' " Luke 3:10-14
People were thinking this dude might be THE dude, so to speak.
John made it clear that he was not and pointed them, instead, to Jesus. What was Jesus's message? Upon emerging from his time of testing in the wilderness after being baptized by John, Matthew 4:17 says, "From that time Jesus began to preach saying, Repent, for the kingdom of heaven is at hand."
After Jesus's ascension into heaven in the clouds, he told his disciples to wait for the power that was to come from on high.
When that power came in the person of the Holy Spirit, there was a very first sermon preached by Peter. What did he say? "...'Let all the house of Israel therefore know for certain that God has made him both Lord and Christ, this Jesus whom you have crucified.' Now when they heard this they were cut to the heart, and said to Peter and the rest of the apostles, 'Brothers, what shall we do?' And 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. For the promise is for you and for your children and for all who are far off, everyone whom the Lord our God calls to himself.' " Acts 2:36-39
Finally, Paul makes it clear in a couple of places in Acts. "The times of ignorance God overlooked, but now he commands all people everywhere to repent, because he has fixed a day on which he will judge the world in righteousness by a man whom he has appointed; and of this he has given assurance to all by raising him from the dead." Acts 17:30-31
He also gave what he viewed was a summary of the various times and places he had presented the gospel in his farewell to the Ephesian elders. "You yourselves know how I lived among you the whole time from the first day that I set foot in Asia, serving the Lord with all humility and with tears and with trials that happened to me through the plots of the Jews; how I did not shrink from declaring to you anything that was profitable, and teaching you in public and from house to house, testifying both to Jews and to Greeks of repentance toward God and of faith in our Lord Jesus Christ." Acts 20:18-21
If repentance plays such a crucial, central, and foundational role in every phase of the gospel's presentation in scripture by John the Baptist, Jesus, Peter, and Paul, how can we feel ANY sense of peace if it is not an extremely prominent role in our thoughts and presentations of the gospel? Is it extremely prominent in your mind? Do you understand that turning from a self centered existence to a Christ centered existence is THE chief fruit that we should all expect to see in our lives if we claim to be born again? I am not telling you this to condemn. This is a sobering reality for me as well. All I ask is that if you cannot honestly say yes to sensing and pursuing a life that is Christ centered, ASK HIM to make that a reality for you. It flows mostly from a genuine appreciation and awareness of his gift of forgiveness and how that genuinely redefines our lives. Again, if you struggle to appreciate that fully (as I do many times), JUST ASK and cry out to Him for the kind of awareness of this that He truly longs for you to have.