Ramsay MacMullen begins his essay “What Difference did Christianity Make?” by citing a question from E. A. Judge:
“What difference did it make to Rome to have been converted?” Self-evident changes like basilica-building or people’s attendance at churches instead of temples are surely not what the question is getting at. The point (or at any rate my point) is rather to discover how broad patterns of secular life changed as a result of the population being now believers. Inquiry promises interesting results because Christianity is known to us as a religion, along with Judaism and certain others, that offers powerful prescriptions for living this secular life. There is a Christian morality, in short; and the introduction of the new faith should thus have had historical impact. [“What Difference did Christianity Make?”, Historia, 35 (1986), 322]
MacMullen looks at the period from A.D. 312 to about A.D. 410 for evidence to marshal in the answering of this question, as he suggests that Christian influence upon society, to be relevant to Judge’s question, must be visible prior to the religion’s becoming a majority, when there is no longer something else it can easily be said to resist and change.
The sources themselves are too often “bookish” (such as Clement of Alexandria), drawing much on pagan themes, while we have no idea to what degree said authors (and preachers) influenced their hearers and readers. If influence on action cannot be demonstrated, moral literature reduces “to the compass of a pastime.” [“Difference”, 323]
Doing, and not just talking, and doing “in some opposition to evidently accepted standards”, is the litmus test of difference. ‘Without opposition [Christianization] cannot have produced any difference.” The standards for what constitutes difference must come from this time. [“Difference”, 324] MacMullen looks at five areas of potential change: (1) Slavery, (2) Attitudes to Sex, (3) Theatrical and Gladiatorial Shows, (4) Judicial Savagery, and (5) Corruption.
No Christian emperor attempted to “abolish” or “even mitigate” slavery “beyond the measures that flowed naturally from prior models.” [324, 325] Constantine forbade senators to have sexual relations with slaves, but the goal was to force them to marry and have legitimate heirs to “continue the line of service to their communities.” [324, fn.7] Constantine banned the splitting up of slave families, but seemingly only on his estates, and trimmed back slave rights.
Life was probably not easier for slaves in Christian times. The Church as an institution, priests as individuals, its heroes and wealthier members all “continued to own, buy, and sell men, women and children.”  Penalties for “peccant” slaves were worse. This is no change in Christianity, no cooling of fervor. From the second century we see Athenagoras writing that some Christians have “many slaves”, and Thecla, in the Acts of Paul and Thecla is “followed about by a large retinue of slaves.” [325, fn.9]
Sex was a prohibited subject in Clement’s world for pagans and Christians, “censured only through its secondary or tertiary connections”  like dress and social habits. The acceptable customs of a region were observed by pagans and Christians alike. The population addressed in these exhortations is largely upper-class.
Codes of propriety varied from place to place.  Thus, women were veiled in third-century pagan Syria, and Lycian Christian women could express suprie that their hostess isn’t wearing makeup.
“High life has its own code.”  Upper classes stick together. Pope Victor secured the release of arrested upper-classfolk through the “God-loving concubine of Commodus, Marcia.” Divorce and remarriage sometimes happened among the elites as well, Christians included, despite Jesus’ teachings (Jerome defended one such case).
Regardless of any difference in pagan and Christian mores on slavery and sexuality, there was much overlap, and no consistent regional and class inflection. There were still special protections of and expectations on upper-class women regarding sex. 
Homosexual love (an upper-class phenomenon?) was unusual, and Dio Chrysostom gently poked fun at it; yet claims to the “unnaturalness” of homosexuality by pagans like Dio lacked the sense that there was a divine intention behind nature. Nonetheless, the demise of this as an open possibility is not Christian in origins: in Roman society, open homosexual relations cannot be found “after Hadrian”. 
Pliny wrote against “clowns, ballet-boys, and mimes”; a noblewoman he knew had “her own private troupe”, yet would dismiss her son before they performed, because she did not entirely approve. Games have an ‘ill effect” on citizens, Pliny argued, and should be abolished. Dio Chrysostom notes the “common ruin [they bring] to whole cities”, both emphasizing the effects that spectacles have on the spectators, the former infecting the latter with an “utter lack of dignity and decency.”  Clement writes in the same vein on these issues. Nazianzen and Basil write similarly on theatrical shows. “Clearly the views [of such] […] had not yet prevailed” after four centuries. “Critics remained […] a censorious minority”. The Christian emperors supplied these shows, as did the pagan Symmachus. “Christianity in this respect had thus made no difference.”
The termination of gladiatorial combat began in “Gaul, Germany, and Britain in the course of the 200s.”  Mostly it seems to have resulted both from shifting tastes in entertainment and the poor return-on-investment of training gladiators (who were expensive, and only survived ten fights on average). Thus, the “role of Christianity [in abolishing] […] gladiatorial combat was nil.” Despite a law from 325, contests were sponsored and/or approved by Constantine, Constantius and Theodosius. Not until the 430s were they finally outlawed, seemingly from “some other force” than Christianity.
There “seems to be no clear pagan-Christian difference”  in the objections of critics; interestingly, neither pagans nor Christians express any “pity for the combatants.”
Despite this absence of expressed pity, there is hardly a mention “of criminals sentenced to the games” , even with the rise of “judicial savagery.” This savagery “represented and produced” changes in career-paths and the way people approached the government.
Examples of this savagery: those who persuaded a young girl to have sex with one who desired her had their throats filled with molten lead; “venal bureaucrats” are warned, then their hands are cut off; kidnappers are stabbed and sent to the amphitheatre; some criminals are consigned to leather sacks full of snakes and drowned; if women owing taxes are treated opportunistically by men they are to be killed by “exquisite tortures”; slaves who snitch on their masters are to be crucified. These laws are “fairly representative” . This trend begins with “the legislation of [Constantine’s] immediate predecessors” (on display in the “Great Persecutions”), though Constantine does “extend” this. The number of capital crimes increases, usually by decapitation. No contemporary of this time ascribes this to a religion, “nor should we.”  It is difficult to find a cause for this savagery in Christianity. The “developing Christian vision of purgatory”  is the “only evidence”  proposed, as it is “the only sadistic literature he is “aware of” from that period, in which a connection is drawn between the elaborate infliction of pain and the will of God.”  Both for pagans and Christians, “cruelty” was produced “in the service of zeal.” As Christians saw cult and morality as elements of religion, the death penalty “spread over many new categories of offense.”
Christians do “display a moral defiance toward law which pagans do not.” Religion “rises above law”, contesting it, but also justifying some kinds of violence and theft.
It is silly to place “responsibility for everything”  at the feet of the Church on the assumption that the Empire became “officially” Christian after 312. There were forces that 312 set in motion, but “we cannot expect a Christian world in the moral sense where there happened merely to be a Christian emperor and widely prominent churches.” The administrative corruption of the fourth century –“a luxuriant jungle of shakedowns, bribes, intimidation, perquisites and privileges”– “traced a line entirely independent of ecclesiastical history” , although all of this was forbidden by the moral standards of earlier centuries.  The emperors had to set “legal limits” to this — but not outright prohibition. After all, it was not seen as an evil. Bribery, for instance, was seen as “good for everybody and good for God, too”, as one Egyptian soldier put it.
This corruption did not only occur among the nominal and the half-converted. One of Pachomius’ disciples was sent to find grain during a famine. He got a deal by buying it “illegally from the public tax stores of a nearby village” , but Pachomius’ only upset was that the disciple bought more than necessary. Basil of Caesarea and the bishop of Iconium were shopping for the best person to bribe to release someone from civic duties. The bishop of Laodicea sought to bribe “high courtiers”. Petronius Probus took offices often to appoint his relatives and friends to posts — including Ambrose of Milan. Augustine looked to secure powerful friends, and his words appear to suggest that bribery was needed for a word of recommendation from these friends. Later, in his sermons, he would endorse bribery for public appointment as fine. He drew “the line at buying the judge and the witnesses” . The bishop of Antioch “involved other bishops in the routine purchase of church office[s].” John Chrysostom excused the bishops of Asia who bought their office, because of how difficult the purchases were on them personally. Christianization made no difference, here. John, Basil, Ambrose and Augustine were all against corruption, in principle.  At least the bishops were wroth with their priests when the latter were guilty of extortion. 
In these areas, “non-Christian moral history runs parallel to Christian. Or the two are one.”  MacMullen cites the bishops at the Council of Serdica cutting each other up with swords, and other examples of monastic or clerical violence. [342, fn.71]There were individuals who rose above the “everyday norms of actual behavior. We know them by their words, which inspire. If we look to deeds, however, and try to see patterns of action in the population at large that clearly reflect Christian preaching, we are hard put to find anything very significant. Of that most aspiring virtue, charity to the point of loving one’s enemies — hardly a sign.”