Did any Early Church Father believe that being armed/self-defence was justified?

To start with Jesus, just after the Last Supper, he not only "allowed" but commanded:

"He who has no sword, let him sell his garment and buy one." ...So they said, “Lord, look, here are two swords.” And He said to them, “It is enough.” (Luke 22:36-38)

Traditionally the idea that the swords were for self-defense is dismissed on the basis of the intervening verse:

For I tell you, this scripture must be fulfilled in me, “And he was counted among the lawless”; and indeed what is written about me is being fulfilled.

However, others speculate that Jesus may have indeed had self-defense in mind, as soldiers were about to arrest him in Gethsemane. If the apostles had not fallen asleep, "two swords" might indeed have been enough to allow Jesus to make his escape.

Liberation theologians go farther, suggesting that he meant the following literally:

Do not think that I have come to bring peace to the earth; I have not come to bring peace, but a sword. (Matthew 10:34)

Whatever the historical Jesus intended, the early church opted not to participate in the Jewish Revolt of 66 c.e., although it may be the case that some Jewish Christians did join.

Church Fathers

Turing to the Church Fathers, the early ones are not known to specifically endorse the use of weapons of self defense. However, the matter is debated. If self-defense includes serving in the army or police forces then the answer must be a qualified "yes." Christians were taught to obey local authorities, and they definitely served in the imperial army. At least one Church Father, Tertullian was quite specific about this:

Marcus Aurelius... bears witness that the Germanic drought was removed by the rains obtained through the prayers of the Christians, who happened to be fighting under him... We (Christians) are sailors along with yourselves; we serve in the army. (Apology)

This does not mean, however, that Tertullian encouraged Christians to fight. In other writings he said:

• Christ, in disarming Peter, disarmed every soldier.

• Shall we carry a flag? It is a rival to Christ.”

• It is absolutely forbidden to repay evil with evil.”

• Only without the sword can the Christian wage war: the Lord has abolished the sword.

Other anti-weaponry quotes from the Church Fathers may be found in the online article Early Church Fathers on refusal of the sword

When we get to the era of the church under Constantine, however, the attitude shifted. The new Christian emperor was himself a military commander, although he delayed baptism until near the time of his death, to avoid the likelihood of post-baptismal sin. Augustine was the first to urge Christians to use force against "heretics" who considered themselves better Christians than the Catholics.

The wandering of the sheep is to be corrected in such wise that the mark of the Redeemer should not be destroyed on it... We have shown that Paul was compelled by Christ; therefore the Church, in trying to compel the Donatists, is following the example of her Lord ...He said to them, "Go out into the highways and hedges, and compel them to come in." In those, therefore, who were first brought in with gentleness, the former obedience is fulfilled; but in those who were compelled, the disobedience is avenged. (The Correction of the Donatists: Chapter 6)

By the time of Pope Gelasius in 494 c.e., the Two Swords mentioned by Jesus had become the Church and the State, one spiritual the other temporal and wielding military power, especially in the struggle against paganism, as well as against heresy when necessary:

There are two powers, august Emperor, by which this world is chiefly ruled, namely, the sacred authority of the priests and the royal power. Of these that of the priests is the more weighty, since they have to render an account for even the kings of men in the divine judgment. You are also aware, dear son, that while you are permitted honorably to rule over human kind, yet in things divine you bow your head humbly before the leaders of the clergy and await from their hands the means of your salvation.