In response, Peter said, repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost. ~ Acts 2:38 

Peter did not tell them, “your sins are already forgiven.” He instructed them to repent for the remission (forgiveness) of sins. 

In Acts 8, Peter confronted Simon the sorcerer because he had offered money in exchange for empowerment to give people the Holy Ghost.

Thy money perish with thee, because thou hast thought that the gift of God may be purchased with money. Thou hast neither part nor lot in this matter: for thy heart is not right in the sight of God. Repent therefore of this thy wickedness, and pray God, if perhaps the thought of thine heart may be forgiven thee. For I perceive that thou art in the gall of bitterness, and in the bond of iniquity. ~ Acts 8: 20-23

Peter did not tell Simon the sorcerer that his sins were already forgiven. Instead, Peter tells him to repent.  

God is a loving and forgiving God, and he desires to forgive people of their sins, but they must turn away from their sins and turn to him to be forgiven. Consider the words of God through the prophet Zechariah:

Therefore say thou unto them, Thus saith the Lord of hosts; turn ye unto me, saith the Lord of hosts, and I will turn unto you, saith the Lord of hosts.

Be ye not as your fathers, unto whom the former prophets have cried, saying, Thus saith the Lord of hosts; Turn ye now from your evil ways, and from your evil doings: but they did not hear, nor hearken unto me, saith the Lord. ~ Zechariah 1:-4

To be forgiven we must turn from our sins.

When Jesus appeared to the apostle Paul, he told Paul that he was sending him to the Gentiles “…to open their eyes, and to turn them from darkness to light, and from the power of Satan unto God, that they may receive forgiveness of sins, and inheritance among them which are sanctified by faith that is in me (faith in Jesus).” ~ Acts 26:17-18

Paul interprets this commission by Jesus as preaching repentance:

Whereupon, O king Agrippa, I was not disobedient unto the heavenly vision: But shewed first unto them of Damascus, and at Jerusalem, and throughout all the coasts of Judaea, and then to the Gentiles, that they should repent and turn to God, and do works meetfor repentance. ~ Acts 26:19-20

Paul was sent the Gentiles to turn them to the Lord. Paul did this through preaching salvation in Jesus Christ through repentance and faith.

Ye know, from the first day that I came into Asia, after what manner I have been with you at all seasons, Serving the Lord with all humility of mind, and with many tears, and temptations, which befell me by the lying in wait of the Jews: And how I kept back nothing that was profitable unto you, but have showed you, and have taught you publicly, and from house to house, Testifying both to the Jews, and also to the Greeks, repentance toward God, and faith toward our Lord Jesus Christ. ~ Acts 20:18-21

To claim that Paul or any other New Testament writer taught forgiveness from God without repentance, is scripturally in error.

In Acts 13, Paul preached to the people and reminded them how John the Baptist had first preached repentance to all the people of Israel ~ v.24

When Paul was in the city of Athens (Acts 17), he was stirred in his spirit because he saw that the whole city given over to idolatry. When Paul found that they had made an altar with the inscription: to the unknown God. Paul used this as an opportunity to preach the gospel to them. Paul’s message can be read beginning in verse 22. In verses 30-31 we read the following:

And the times of this ignorance God winked at; but now commandeth all men everywhere to repent: because he hath appointed a day, in the which he will judge the world in righteousness by that man whom he hath ordained; whereof he hath given assurance unto all men, in that he hath raised him from the dead. ~ Acts 17:30-31

Notice that Paul says that God has commanded “all men, everywhere to repent” and Paul says this within the context of the coming Day of Judgment, wherein God is going to judge men through Jesus Christ.

While at Lystra, Paul and Barnabas preached the gospel to the people and a man who had been crippled all his life was healed. This caused quite a stirring among the idolatrous people at Lystra who then attempted to worship Barnabas and Paul.

And when the people saw what Paul had done, they lifted up their voices, saying in the speech of Lycaonia, The gods are come down to us in the likeness of men. And they called Barnabas, Jupiter; and Paul, Mercurius, because he was the chief speaker. Then the priest of Jupiter, which was before their city, brought oxen and garlands unto the gates, and would have done sacrifice with the people. Which when the apostles, Barnabas and Paul, heard of, they rent their clothes, and ran in among the people, crying out, And saying, Sirs, why do ye these things? We also are men of like passions with you, and preach unto you that ye should turn from these vanties unto the Living God, which made heaven, and earth, and the sea, and all things that are therein… ~ Acts 14:11-15

Notice that Barnabas and Paul preached to the people to turn from their idolatrous practices and serve the living God. Turning from idolatry to serve the Living God is repentance..

In Acts 15, the Jerusalem counsel (of which Paul and Barnabas were a part) concluded that the Gentiles in Antioch were brethren in Christ because they had turned to God.

Known unto God are all his works from the beginning of the world. Wherefore my sentence is, that we trouble not them, which from among the Gentiles are turned to God: But that we write unto them, that they abstain from pollutions of idols, and from fornication, and from things strangled, and from blood.  ~ Acts 15:18-20

In Romans 11:26 Paul refers to Jesus as the deliverer who shall come out of Zion and shall turn ungodliness from Jacob. Paul also says the following to the Thessalonians:

For from you sounded out the word of the Lord not only in Macedonia and Achaia, but also in every place your faith to God-ward is spread abroad; so that we need not to speak anything. For they themselves show of us what manner of entering in we had unto you, and how ye turned to God from idols to serve the Living and true God; And to wait for his Son from heaven, whom he raised from the dead, even Jesus, which delivered us from the wrath to come. ~ 1 Thessalonians 1:8-10

Throughout the entire Bible, forgiveness is the blessing that come through repentance.

The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance. ~ 2 Peter 3:9