We all need Jesus to open our understanding so that we may understand the scriptures.

"And he said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me.

“Then opened he their understanding, that they might understand the scriptures," - Luke 24:44-45 KJV

Jesus had told His disciples many times that He must suffer and die and the third day rise again according to the scriptures. Yet, His disciples did not understand.

"From that time forth began Jesus to shew unto his disciples, how that he must go unto Jerusalem, and suffer many things of the elders and chief priests and scribes, and be killed, and be raised again the third day." - Matthew 16:21 KJV

How did Peter react?

"Then Peter took him, and began to rebuke him, saying, Be it far from thee, Lord: this shall not be unto thee." - Matthew 16:22 KJV

And what was Jesus’ response?!

"But he turned, and said unto Peter, Get thee behind me, Satan: thou art an offence unto me: for thou savourest not the things that be of God, but those that be of men." - Matthew 16:23 KJV

Peter needed his understanding opened! Jesus had just told Peter He would give him the keys to the kingdom of heaven, but at this point, Peter didn’t understand.

Jesus had a purpose: He put on human flesh so that He could die, to shed sinless blood to pay for the sins of man.

"Forasmuch then as the children are partakers of flesh and blood, he also himself likewise took part of the same; that through death he might destroy him that had the power of death, that is, the devil; and deliver them who through fear of death were all their lifetime subject to bondage." - Hebrews 2:14-15 KJV

On the Day of Pentecost, when Jesus poured out His Spirit on all those in the upper room, Peter and the rest of the disciples had their understanding opened. They understood that Jesus fulfilled the scriptures concerning His death, burial, and resurrection.

Under the anointing of the Holy Ghost, Peter preached the first gospel message how Jesus fulfilled the scriptures by His death, burial, and resurrection. Now Peter understood the keys Jesus gave to him.

After preaching the gospel, he answered the crowd how to obey the gospel, the keys to the kingdom of heaven.

"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. ... Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls.

“And they continued stedfastly in the apostles' doctrine and fellowship, and in breaking of bread, and in prayers. ... Praising God, and having favour with all the people. And the Lord added to the church daily such as should be saved." - Acts 2:38, 41-42, 47 KJV