"I am the gate; whoever enters through me will be saved." John 10
There are two paintings on this subject in a similar style.
Both of them are feature key details including the crown of thorns, the dove (a symbol of the Holy Spirit) and the deer (a symbol of devotion and longing for God). Through the gate you can see Paradise and the Holy Mountain.
The second painting is more detailed and features a series of scenes positioned around the central gate. These scenes are all key moments leading up to the crucifixion of Christ: Judas betraying Jesus with a kiss, Jesus restraining the apostles from fighting the soldiers, Pilate washing his hands, and Saint Peter's desolation on hearing the cock crowing.
These emphasise the fact that it is only through Jesus' sacrifice that we are saved. In John 10 Jesus continues, " I have come that they may have life"; through his resurrection we are all able to join him in eternal life.
Peter has used this compositional idea - combining many scenes into one picture - in other paintings including Christ the Teacher, Cross of the Eucharist and the Prodigal Son.