To say that Constantine's conversion was "the single biggest act" that helped spread Christianity is an over simplification, and essentially false. Such a statement suggests several assumptions which can be thoroughly proved false (given the sources available). Those assumptions are as follows:
1. Christianity was weak and insignificant prior to Constantine.
2. Constantine's own conversion was complete, and his character was such that any other emperor in his same situation wouldn't have done the same.
3. Constantine's conversion drastically altered Rome's policy towards Christianity.
So lets take these one at a time;
1. Christianity was weak and insignificant prior to Constantine.
Despite the best efforts of Rome, Christianity had clearly established a power base prior to the reign of Constantine (r. 272-337 ce). Roman persecution of Christianity in its initial centuries is well documented, particularly under the so called "bad emperors" such as Caligula (r. 37 - 41 ce) and Nero (r. 54 - 68 ce). During Nero's reign historian Tacitus, writing in 64 c.e. addresses the situation as follows:
In spite of this brutal treatment at the hands of the Emperor, Tacitu's makes evident that Christianity was spreading at an impressive rate, even achieving a noticeable population in the "Eternal City" itself as early as the 60's ce.
Less than 50 years later we already begin to see a shifting attitude towards the increasing Christian population in the Empire. In the year 112, Pliny the Younger, governor in the Roman province of Bithynia (modern day Turkey), faced a number of Christians in his court. It is unclear what the initial charges are, but he ultimately decided, despite the fact that the Christians seemed generally harmless to him, that he should execute them if they refused to recant their faith. Because he is unsure as to whether he can kill them legally for no other crime than their faith, he writes to the Emperor for advice. (Pliny's original letter can also be seen at the previous link). The Emperor Trajan's response marks an already sharp turn in Roman policy:
While Trajan does affirm Pliny's actions, he also instructs him to avoid actively hunting Christians and to allow convicted Christians every opportunity to recommit themselves to Roman gods. If you consider the Roman affinity for custom and tradition, this is a very drastically different approach than 50 years prior.
Additionally, at a time when correspondence between a provincial governor in Anatolia and the Emperor in Rome was far from easy, why would Pliny waste time worrying about Christians in his province if they were, to that point, and insignificant population?
To this effect, by the year 200 ce Christianity was well established in the eastern half of the Roman empire.
It is also worth noting that by 200 ce the eastern half of the empire was quickly becoming the economic and cultural base of Rome. So much so, in fact, that by Constantine's reign at the the turn of the 4th century he moved the capitol of the Roman Empire from Rome to his own, new city of Constantinople (modern day Istanbul) om the European side of the Turkish Straights.
2. Constantine's own conversion was complete, and his character was such that any other emperor in his same situation wouldn't have done the same.
It is well documented that even Constantine himself never truly committed to Christianity. This can be proved quickly:
Here is a coin minted 5 years after Constantine's conversion. The perfectly pagan deity Sol Invictus can be readily identified standing on the reverse side of the coin (
Sol Invictus - Wikipedia, the free encyclopedia). In fact, Christ was simply another Diety in the polytheistic pantheon sponsored by Constantine.
This bring's into question Constantine's motives for conversion. At the time of his conversion Constantine was embroiled in a bloody civil war against the other would-be emperor Maxentius. In the year 312 Constantine seemed to be on the losing end of this struggle when, according to him, he saw a vision of the Greek
chi rho symbol, which he interpreted as a message from Christ to accept him as a god. Now, If you would like to believe that fine, however the more logical situation would be that Constantine was making a play to drum up support from the increasingly substantial Christian population in the eastern half of the empire.
Had his conversion been genuine he surely would have followed its first commandment,
Thou shalt have no other gods before me, Which he clearly did not, as proven by the coin above, and the fact that Constantine was deified after his death.
3. Constantine's conversion drastically altered Rome's policy towards Christianity.
As we have seen in Trajan's response to Pliny in the 2nd century, Roman persecution of Christianity was already beginning to lose intensity. Under the reign of Constantine, over 150 years later, all that was really accomplished was the Edict of Mediolanum in 313, which states:
In other words, tolerance of ALL RELIGIONS was granted under Constantine.
The fact that Christianity continued to spread was indicative more of its own momentum, rather then the conspicuous conversion of Constantine. It seems likely that if any other religion had significantly taken hold in the Empire it would have been that religion which Constantine converted. Even still, the religion still didn't quite take hold amongst the notoriously ultra-conservative Romans:
