Philadelphia and Smryna are the only churches that did not receive warnings from the Lord in the seven letters recorded in Revelation. Philadelphia had "a little strength" because they had built their church on the two foundations of the Word of God and the name of the Lord Jesus.
The foundation of Jesus Christ Himself (1 Corinthians 3:11) and the foundation of the writings of the "apostles and prophets" (Ephesians 2:20) which are inspired by God (2 Timothy 3:16) make the church "the pillar and ground of the truth" (1 Timothy 3:15). Philadelphia had faithfully held these eternal principles and were therefore given an "open door."
The Lord’s introduction to Philadelphia cites the "Key of David," suggesting a reference to the treasure house of the king (1 Kings 7:51) and to Christ’s authority as the heir to the kingdom (Isaiah 22:22). The treasure of the eternal kingdom is not physical riches, but the gold, silver, and precious stones of God ordained work for the kingdom (1 Corinthians 3:12-13).
But just as the talents and the pounds granted to the servants in the parables (Matthew 25; Luke 19), the open door is an opportunity to use the resources of the King for His benefit--not a guarantee of success. The Lord grants the resources, but the work and the use of those resources are our responsibility. We will be held accountable.
If we use those resources well, even those of the "synagogue of Satan" will "come and worship" (Revelation 3:9) and "every tongue |will| confess that Jesus Christ is Lord, to the glory of God the Father" (Philippians 2:11). HMM III