In the letter to the sixth church, Jesus declares that he is holy (set apart, different and special) and true (genuine and faithful). He declares that he has the key of David. David was a king. He had a heart for God unlink any of Israel's other kings. And, as he was king, he had the authority to make sure something happened ... by opening up a way with his "key." Or he he had the authority to make sure something did not happened ... by closing a way with his "key."
This is a small church without much influence. and they are in a place where they are being opposed by Satan.
Even though they are small and without much influence, they are obedient. And God recognizes their work. So, he uses his "key" to open a door for them. They have a special opportunity available to the, and no one will be able to stop them. Those who oppose this fmall part of God's family will be forced to admit that God loves them and works on their behalf. And, as they have prepared their hearts to patiently endure oppression against them, God will protect them against the coming persecution.
And if these children continue to live with this mind-set of obedience and faithfulness, God will bless them with a special relationship. They will always be in his presence. They will be marked in a special way as citizens of his kingdom, and members of his family. And they will have a new identity.