And when you hear that we look for a kingdom, you suppose, without making any inquiry, that we speak of a human kingdom. Instead, we speak of that which is with God, as can be shown from the confession of their faith made by those who are charged with being Christians, even though they know that death is the punishment awarded to those who so confess. For if we looked for a human kingdom, we would deny our Christ, so that we might not be killed. We would try to escape detection, so that we might obtain what we hope for. But since our thoughts are not fixed on the present, we are not concerned when men cut us off; since death is a debt which must at all events be paid. (First Apology 11)

Who among the believers does not know the words in Isaiah? “In the last days the mountain of the Lord shall be revealed, and the house of the Lord on the top of the mountains, and it shall be exalted above the hills. All nations shall come to it. Many people shall go and say, ‘Come, and let us go up to the mountain of the Lord, to the house of the God of Jacob, and he will teach us his way, and we will walk in it.” For out of Zion shall go forth a law, and a word of the Lord from Jerusalem. He shall judge among the nations, and shall rebuke many people. They shall beat their swords into ploughshares, and their spears into pruning-hooks; nation shall not lift up sword against nation; neither shall they learn war any more [Isa. 2:1-5]. (Letter from Origen to Africanus 15)

“First of all, sir,” I said, “Explain to me what is the meaning of the rock and the gate?”
“This rock,” he answered, “and this gate are the Son of God.”
“How, sir?” I said. “The rock is old, and the gate is new.”
“Listen,” he said, “and understand, O ignorant man. The Son of God is older than all his creatures, so that he was a fellow counselor with the Father in his work of creation. For this reason he is old.”
“And why is the gate new, sir?” I said.
“Because,” he answered, “he became manifest in the last days of the dispensation. For this reason the gate was made new, that they who are to be saved by it might enter into the kingdom of God.” (Shepherd of Hermas III:9:12)

No Christian can have a doubtful mind on the point that our faith is not in the creature, but in one God, Father Almighty, Maker of all things visible and invisible; and in one Lord, Jesus Christ, his only-begotten Son, and in one Holy Spirit; one God, known in the holy and perfect Trinity, baptized into which, and in it united to deity, we believe that we have also inherited the kingdom of the heavens, in Christ Jesus our Lord. (Synodal Letter to the Bishops of Africa 11)