It is not clear what the door refers to. There is nothing in what Jesus has said earlier in his sermon that helps us to understand his meaning. Possibly it refers to the gate of salvation (Matthew 7:13).1 More likely it is the concluding statement of a triplet that emphasises the need to pray for God’s help and the promise of God’s aid to those who do so. Jesus is training his disciples to bold faith. He wants them to expect God to do big things in answer to their prayers.
7 “Ask, and it will be given to you; seek, and you will find; knock, and it will be opened to you.