What was Jesus saying to Peter when He said, “Whatsoever thou shalt bind on earth shall be bound in heaven: and whatsoever thou shalt loose on earth shall be loosed in heaven." Matthew 16:19?

According to Thayer's Greek Lexicon, 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.

Before Jesus ascended to heaven, Jesus spoke to His disciples. After opening their understanding, He revealed specifically the doctrine Peter was to preach. He was to preach Jesus’ death, burial, and resurrection, and then repentance and remission of sins in Jesus name. Jesus also spoke of the promise of the Father of the power they would receive. This is the gospel.

"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, behold, I send the promise of my Father upon you: but tarry ye in the city of Jerusalem, until ye be endued with power from on high." - Luke 24:49 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

In Jerusalem, on the Day of Pentecost, the disciples all received the promise of the Father – the baptism of the Holy Ghost. (Acts 2:4, Acts 2:33) 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 2:14, 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. For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call." - Acts 2:38-39 KJV

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.

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