Matthew 16:18 And I tell you, you are Peter, and on this rock I will build my church, and the gates of hades shall not prevail against it.
Earlier in the conversation Jesus was having with His disciples, Peter had answered Jesus' question correctly, "You are the Christ, the Son of the living God." Peter had received this knowledge by revelation from God the Father. Peter became the leader of the disciples. Jesus told him that He would build His church on "this rock". In 1 Peter 2:4-5,8 Peter takes "this rock" to be the Christ the "living stone", "a stone of stumbling, a rock of offense." Peter was instrumental in the beginning of the Church, but the Church is built upon "this rock", Jesus Christ (see 1 Corinthians 3:11).
The gates of Hades would be pushed open from the inside. All who died before Christ rose from the dead (with two exceptions), went to Hades, which was divided into two compartments that could not be accessed by the other. One compartment was Abraham's Bosom, where those who believed in the Lord went upon death and the other compartment was the place of torment, which those who didn't believe in the Lord went upon death of his/her body (see Luke 16:19-31). Ephesians 4:8 reveals that the Christ led all the righteous out of Hades into heaven when He rose from the dead and ascended back to heaven. All believers now go straight to heaven upon the death of their bodies (Philippians 1:21-23; 2 Corinthians 5:8). Hades now has one compartment, the place of torment inhabited by unbelievers awaiting the final judgment. Revelation 20:14 instructs us that Hades is cast into the Lake of Fire.
The believer in Christ will be used by Christ to build His Church. The gates of Hades no longer contain the believer in Christ. Every believer now departs to be with Christ when his/her body dies.