It's curious, but the truth is exactly opposite: Constantine is a whip used to beat the Catholic church.
As Roman Emperors go, Constantine was well above average in every category, and nobody much objects to him as an Emperor (well, yes, there is that matter of some family murders, but then what emperor didn't kill off some of those closest to him? </sarc>).
But Constantine did something no other Emperor did: he not only legalized Christianity, but he outlawed Christian heresies (through the Nicene Council), and he began to overturn the old Roman pagan religions.
Constantine called the Nicene Council, paid the bishops' travel & lodging expenses, provided them his great hall, spoke at the council and "exhorted the Bishops to unanimity and concord".
Constantine then enforced the Council's decisions.
In short, Constantine took the first giant steps toward making Christianity the Empire's state religion, and toward making Christian heresies illegal.
And that is the tradition carried forward over a thousand years which confronted Martin Luther and other Protestant reformers.
In Constantine, the Church had made a pact with the devil, and in doing so became something of a devil itself.
The reality is that CA and Catholic Answers Forums has a persecution complex, seeing all "anti-Catholics" in every closet, that being one of its most frequent charges, while its forum is extremely touchy and regularly censures or banns people for slight offenses. I think they miss the powers of the inquisition and this is as close as they can get.
In short, Constantine took the first giant steps toward making Christianity the Empire's state religion, and toward making Christian heresies illegal.
Under Constantine, The so-called "Edict" of Milan of February 313 expressly granted religious liberty to Christians, who had been the object of special persecution, but also grants liberty to all religions:
When you see that this has been granted to [Christians] by us, your Worship will know that we have also conceded to other religions the right of open and free observance of their worship for the sake of the peace of our times, that each one may have the free opportunity to worship as he pleases; this regulation is made that we may not seem to detract from any dignity of any religion." —"Edict of Milan", Lactantius, On the Deaths of the Persecutors (De Mortibus Persecutorum), ch. 48. opera, ed. 0. F. Fritzsche, II, p 288 sq. (Bibl Patr. Ecc. Lat. XI).[8]
But while Constantine tolerated paganism and other religions, he actively promoted Christianity. He called the Council of Nicaea in an attempt to establish an empire-wide orthodoxy and end the controversy with Arianism.
This referenced WP article states,
Constantine took over the role of the patron for the Christian faith. He supported the Church financially, had an extraordinary number of basilicas built, granted privileges (e.g. exemption from certain taxes) to clergy, promoted Christians to high-ranking offices, returned property confiscated during the Great Persecution of Diocletian,[15] and endowed the church with land and other wealth.[16] Between 324 and 330, Constantine built a new imperial capital at Byzantium on the Bosphorus, which would be named Constantinople for him. Unlike "old" Rome, the city employed overtly Christian architecture and contained churches within the city walls and had no pre-existing temples from other religions.[17]
In doing this, however, Constantine required those who had not converted to Christianity pay for the new city.[16] Christian chroniclers tell that it appeared necessary to Constantine "to teach his subjects to give up their rites (...) and to accustom them to despise their temples and the images contained therein,"[18] This led to the closure of temples because of a lack of support, their wealth flowing to the imperial treasure;[19] Constantine did not need to use force to implement this.[16] Only the chronicler Theophanes has added that temples "were annihilated", but this was considered "not true" by contemporary historians.[20]
It was the The Edict of Thessalonica jointly issued by Theodosius I, Gratian, and Valentinian II on 27 February 380 that made the catholic church the state church.
After the edict, Theodosius spent a great deal of energy suppressing all non-Nicene forms of Christianity, especially Arianism, and in establishing Nicene orthodoxy throughout his realm.[4].
In 383, the Emperor ordered the various non-Nicene sects (Arians, Anomoeans, Macedonians, and Novatians) to submit written creeds to him, which he prayerfully reviewed and then burned, save for that of the Novatians. The other sects lost the right to meet, ordain priests, or spread their beliefs.[6] Theodosius prohibited the residence of heretics within Constantinople, and in 392 and 394 confiscated their places of worship.[7]
Today we have the IRS in a much smaller degree being ideologically oppressive.
Constantine favored Christianity as a party but was not a Christian until he was on his deathbed, and he favored Arianism, which was not the faith of the majority because it was closer to his own faith, which was the worship of Sol Invictus. He also considered Judaism, but it was so peculiar, so national that he decided he could not subscribe to it, even though is many respects it was more like the pagan faiths. We see how he and his mother decided to build Christian temples to replace the old temples. She went to Jerusalem and laid the groundwork that led to the construction of the Church of the Holy Sepulcher and a Church of our Lady. If Constantine had chosen Judaism, he would have reconstructed the Jewish Temple.
If it was written by one of The 12, how can it be heresy?