Jesus answered, My kingdom is not of this world: if my kingdom were of this world, then would my servants fight, that I should not be delivered to the Jews: but now is my kingdom not from hence. John 18:36

Thus also, says He, “Those who wish to behold Me, and lay hold of My kingdom, must through tribulation and suffering obtain Me.”Barnabas (A.D. 70-130) ch.7

And you know, brethren, that the sojourn of this flesh in this world is mean and for a short time, but the promise of Christ is great and marvelous, even the rest of the kingdom that shall be and of life eternal. What then can we do to obtain them, but walk in holiness and righteousness, and consider these worldly things as alien to us, and not desire them? For when we desire to obtain these things we fall away from the righteous path. Second Clement (A.D. 100) ch.5

But the Lord said, No servant can serve two masters. If we desire to serve both God and mammon, it is unprofitable for us: For what advantage is it, if a man gain the whole world and forfeit his soul? Now this age and the future are two enemies. The one speaks of adultery and defilement and avarice and deceit, but the other bids farewell to these. We cannot therefore be friends of the two, but must bid farewell to the one and hold companionship with the other. Let us consider that it is better to hate the things which are here, because they are mean and for a short time and perishable, and to love the things which are there, for they are good and imperishable. For, if we do the will of Christ, we shall find rest; but if otherwise, then nothing shall deliver us from eternal punishment, if we should disobey His commandments. Second Clement (A.D. 100) ch.6

But while (The Christians) dwell in cities of Greeks and barbarians as the lot of each is cast, and follow the native customs in dress and food and the other arrangements of life, yet the constitution of their own citizenship, which they set forth, is marvelous, and confessedly contradicts expectation. They dwell in their own countries, but only as sojourners; they bear their share in all things as citizens, and they endure all hardships as strangers. Every foreign country is a fatherland to them, and every fatherland is foreign. Letter to Diognetus (A.D. 125-200) ch.5

He said to me; “You know that you, who are the servants of God, are dwelling in a foreign land; for your city is far from this city. If then you know your city, in which you shall dwell, why do you here prepare fields and expensive displays and buildings and dwelling-chambers which are superfluous?
He, therefore, that prepares these things for this city does not purpose to return to his own city.
O foolish and double-minded and miserable man, do you not perceive that all these things are foreign, and are under the power of another? For the lord of this city shall say, “I do not wish you to dwell in my city; go forth from this city, for you dost not conform to my laws.”
You, therefore who have fields and dwellings and many other possessions, when you are cast out by him, what will you do with your field and your house and all the other things that you prepared for yourself? For the lord of this country said to you justly, “Either conform to my laws, or depart from my country.”
What then shall you do, who are under law in your own city? For the sake of your fields and the rest of your possessions will you altogether repudiate your law, and walk according to the law of this city? Take heed, lest it be inexpedient to repudiate the law; for if you should desire to return again to your city, you shall surely not be received [because you didst repudiate the law of the city], and shall be shut out from it.
Take heed therefore; as dwelling in a strange land prepare nothing more for yourself but a competency which is sufficient for you, and make ready that, whensoever’s the master of this city may desire to cast you out for your opposition to his law, you may go forth from his city and depart into your own city and use your own law joyfully, free from all insult. Hermas (A.D. 150) Ante-Nicene Fathers vol.2 pg. 30

And when you hear that we look for a kingdom, you suppose, without making any inquiry, that we speak of a human kingdom; whereas we speak of that which is with God, as appears also from the confession of their faith made by those who are charged with being Christians, though they know that death is the punishment awarded to him who so confesses. For if we looked for a human kingdom, we should also deny our Christ, that we might not be slain; and we should strive to escape detection, that we might obtain what we expect. But since our thoughts are not fixed on the present, we are not concerned when men cut us off; since also death is a debt which must at all events be paid. Justin Martyr (A.D. 160) Ante-Nicene Fathers vol.1 pg.166