The sole reason some Christians were against military service in Roman Army was false worship of false gods was required.
If you look up sermons in early church on Romans 13, they are pro military service,
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Homily 23 on Romans
Rom. XIII. 1
Let every
soul be subject unto the higher powers.
Of this subject he makes much account in other epistles also, setting subjects under their rulers as household servants are under their masters. And this he does to show that it was not for the subversion of the commonwealth that Christ introduced His
laws, but for the better ordering of it, and to teach men not to be taking up unnecessary and unprofitable
wars. For the plots that are formed against us for the
truth's sake are sufficient and we have no need to be adding
temptations superfluous and unprofitable. And observe too how well-timed his entering upon this subject is. For when he had demanded that great spirit of heroism, and made men fit to deal either with friends or foes, and rendered them serviceable alike to the prosperous and those in adversity and need, and in fact to all, and had planted a conversation worthy of
angels, and had discharged
anger, and taken down recklessness, and had in every way made their mind even, he then introduces his exhortation upon these matters also. For if it be right to requite those that injure us with the opposite, much more is it our duty to
obey those that are benefactors to us. But this he states toward the end of his exhortation, and hitherto does not enter on these reasonings which I mention, but those only that enjoin one to do this as a matter of debt. And to show that these regulations are for all, even for
priests, and
monks, and not for men of secular occupations only, he has made this plan at the outset, by saying as follows: let every
soul be subject unto the higher powers, if you be an Apostle even, or an Evangelist, or a Prophet, or anything whatsoever, inasmuch as this subjection is not subversive of religion. And he does not say merely
obey, but be subject. And the first claim such an enactment has upon us, and the reasoning that suits the faithful, is, that all this is of God's appointment.
For there is no power, he says, but of
God. What say you? It may be said; is every ruler then elected by
God? This I do not say, he answers. Nor am I now speaking about individual rulers, but about the thing in itself. For that there should be rulers, and some rule and others be ruled, and that all things should not just be carried on in one confusion, the people swaying like waves in this direction and that; this, I say, is the work of God's wisdom. Hence he does not say, for there is no ruler but of
God; but it is the thing he speaks of, and says, there is no power but of
God. And the powers that be, are ordained of
God. Thus when a certain wise man says, It is by the Lord that a man is matched with a
woman Proverbs 19:14,
Septuagint, he means this, God made marriage, and not that it is He that joins together every man that comes to be with a
woman. For we see many that come to be with one another for
evil, even by the law of marriage, and this we should not ascribe to
God. But as He said Himself, He which made them at the beginning, made them male and female, and said, For this
cause shall a man leave father and mother, and shall cleave to his wife, and they two shall be one flesh.
Matthew 19:4-5;
Genesis 2:24 And this is what that wise man meant to explain. For since equality of
honor does many times lead to fightings, He has made many governments and forms of subjection; as that, for instance, of man and wife, that of son and father, that of old men and young, that of bond and free, that of ruler and ruled, that of master and
disciple. And why are you surprised in the case of
mankind, when even in the body He has done the same thing?"