Jesus said,

"And I say also unto thee, that thou art Peter, and upon this rock I will build my church; and the gates of hell shall not prevail against it. And I will give unto thee the keys of the kingdom of heaven: and whatsoever thou shalt bind on earth shall be bound in heaven: and whatsoever thou shalt loose on earth shall be loosed in heaven." - Matthew 16:18-19

The word “bind” in this verse means, “to forbid, prohibit, declare to be illicit,” while the word “loose” in this verse means, “to declare lawful.” In other words, it appears that Jesus in this verse is giving authority to Peter to establish doctrine.

After Jesus’ death, burial, and resurrection and before He ascended to heaven, Jesus spoke to His disciples.

After giving Peter the authority to establish doctrine in Matthew 16:18-19, Jesus opened the apostles' understanding and revealed specifically the doctrine Peter was to preach.

"Then opened he their understanding, that they might understand the scriptures, and said unto them, Thus it is written, and thus it behoved Christ to suffer, and to rise from the dead the third day: and that repentance and remission of sins should be preached in his name among all nations, beginning at Jerusalem." - Luke 24:45-47 KJV

"And, being assembled together with them, commanded them that they should not depart from Jerusalem, but wait for the promise of the Father, which, saith he, ye have heard of me. For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence." - Acts 1:4-5 KJV

On the Day of Pentecost, filled with the Holy Ghost, Peter, standing up with the other eleven apostles, lifted up his voice and declared to the multitude that had gathered the gospel: the death, burial, and resurrection of Jesus. (Acts 2:4, Acts 2:14, Acts 2:23-24)

Peter, with the keys of the kingdom of heaven and authority, both of which were given to him by Jesus, declared as doctrine how we must obey the gospel:

"Then Peter said unto them, 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, in accord with the other eleven apostles, established on this day the doctrine (law) of salvation for the new testament church, Acts 2:38.