We can neither see nor judge a man's heart. We can look at some of what he did and ponder if these are the actions of a Christian, but whether he really was saved is something God only knows.
Interesting to note:
When we are saved we have been changed; we are changing; and we will someday be changed... because we were saved (past tense), we are being saved (present and ongoing sense) and we will be saved (future tense which points to our final glorification)
We are perfect on day two of being saved, we aren't even perfect on day 1,000... we are simply works in progress with a new heart...
Well, there are compelling reasons to believe in the salvation of those saints venerated by Catholics, traditional Protestants, and especially the Orthodox (whose process of Glorification, which is the Eastern and Oriental Orthodox term for what the Roman Catholics call canonization, strikes me as more robust, and also more scriptural, in that it immediately recognizes martyrs as saints, that is to say, saved, as per the words of our Lord, “He who confesses me before men I shall confess before the Father” - it is an ancient principle of the early church that all martyrs of the true Christian faith who die for confessing Jesus Christ as Lord, and to a large extent, those who were tortured for confessing Him as Lord, with some exceptions*, are saved, even if not baptized, even if they confess their faith in Jesus Christ in extremis, which is to say at the moment of death, it is said they are baptized by their spilt blood, and receive a crown of martyrdom as much as anyone else who is killed for confessing Christ as Lord and Savior, for Christ is God Incarnate, the second person of the Trinity, the only begotten son and word of God, who together with the Father and the Son is alone worthy of veneration).
Now, all churches who venerate saints, which include but are not limited to the Eastern and Oriental Orthodox, the Roman Catholics, the Anglicans, the Lutherans, the Methodists, and the Assyrian Church of the East, will concede the vast majority of heavenly saints, that is members of the faithful who are saved, are unknown, because there are so many pious Christians, and for people to be aware of someone whose circumstances of life and death are indicative of salvation and thus heavenly sainthood (theosis, as the Orthodox call it, or Entire Sanctification as Wesley calls it), those circumstances generally have to be exceptional. That is why the Anglican, Orthodox, Assyrian and Catholic churches, among others (I think the Methodists and Lutherans as well, but I am not sure, perhaps
@Methodized and
@MarkRohfrietsch could answer that), have a feast of All Saints, which in the Eastern Orthodox Church is the first Sunday after Pentecost (which is regarded by the EO as a Trinitarian feast, because of the sending by the Son of the Holy Spirit who proceeds from the Father, to serve as our Paraclete) so Trinity Sunday in the Byzantine liturgy would be superfluous), and in the Anglican and Catholic churches is November 1st.
These five ancient communions (RC, EO, OO, Assyrian and high church Anglicans) also have the custom of praying to the saints for their intercession, especially the Virgin Mary, owing to her status established by the Council of Ephesus as Theotokos, and praying for the souls of those not known to be saints, but not anathematized.
In the case of St. Constantine, it seems more likely than not that He is among the members of the Church Triumphant, because he is venerated by all of the ancient churches,** and he did stop the Diocletian persecution.
This particular act, and its importance to Christianity, cannot be overstated, because the persecution of Christians by Diocletian was the worst experienced by Christianity under the Roman Empire, and would remain unsurpassed by any subsequent persecution with the exception of the wholesale genocide of the Assyrian Christians in China, Tibet, Mongolia, Yemen and Central Asia by Tamerlane, and the genocide of Christian converts in Japan and China, and the genocide of North African Christians in the Sudan, Libya, Africa Proconsularis (Tunisia), Algeria, and Morocco, and the barbaric Turkish genocide against Armenians, Syriac and Assyrian Christians, and the recent attempted ethnic cleansing and genocide in Iraq and Syria, by various Islamic caliphates. But aside from Islam, and the case of Japan and China, the Diocletian persecution is the worst that has happened, with children such as St. Abanoub, a 12 year old boy, martyred alongside elderly bishops like St. Paul, the Patriarch of Alexandria and the Church in Egypt.
It was objectively worse than the Communist persecution in all places except Albania (which was the only Communist country to try to suppress all religions) and possibly some of the Asiatic Communist states. But certainly the persecution of Christians in the Soviet Union, the Warsaw Pact, Yugoslavia and Cuba paled in comparison to the Diocesan Persecution, which had as its goal the conversion of all Christians in the Empire to Paganism, and the extermination of those who refused, whereas the aforementioned Communists did persecute the Church, but did not try to wipe it out (unlike Enver Hoxha, the dictator of Albania, who failed spectacularly, by the way, because Albanian Christianity came back flourishing along with other Albanian religions after the downfall of his regime, aided by the Albanian diaspora in America and elsewhere, which notably included Mother Theresa). The Communists realized it was more expedient to try to subvert the church, using it as a tool for external propaganda and internal surveillance, by recruiting, using blackmail or extortion, members of the clergy, into acting as informants for the KGB, the Stasi, the Securitate and other dreaded secret police in the Chekist tradition. Also during WWII Stalin needed the support of the Orthodox bishops to mobilize the population to repel the Nazi invasion.
So, the importance and extreme virtue of Emperor Constantine stopping the persecution initiated by his predecessor Diocletian, and using military force to remove those of his co-emperors under the Tetrarch system Diocletian initiated in his various reforms who continued to persecute Christians, cannot be overstated.
It was also not uncommon in that era for Christians to remain catechumens until they were near to death, because of rigorist errors expressed by heretics like Tertullian who believed that sins would not be forgiven among the baptized, and since baptism washes away all sins, one would assume that Constantine, fearful of what he might be required to do as Emperor and guilty over his previous actions, did not deem himself worthy to receive baptism and the Eucharist while still in good health, and wanted to definitively repent of his wrongdoing at the end of his life. The Early Church was later able to stamp out this unhealthy rigorism.
So, I would argue that Constantine is almost certainly a saint, for stopping the Diocletian persecution, repenting of it, and promoting Christianity as the preferred religion in the Empire, and for his initial support of the decisions of the Council of Nicea.
*Origen Adimantius, the great third century Alexandrian theologian, who was compelled to emasculate himself when he refused to offer incense to pagan idols during one of the persecutions of that century, was later anathematized by the Chalcedonian churches by Emperor Justinian, along with Theodore of Mopsuestia, acts ratified by the Fifth Ecumenical Council, which were at the time so unpopular they caused a schism in the Western Church known as the Three Chapters Controversy, however, to my knowledge, he was not anathematized by the Oriental Orthodox or the Assyrians. Many regard the anathemas of Theodore of Mopsuestia and especially of Origen to have been improper, since both men had died in the peace of the church, not having caused schism or having been accused of heresy in their life, and indeed were widely venerated as saints (Theodore of Mopsuestia still is venerated by the Assyrians).
** There are cases where only one or two of the ancient churches venerates a particular saint, and these in a minority of cases are dubious, or disputed by other churches. For example, only the Assyrians venerate Nestorius, and only the Roman Catholics venerate Ignatius de Loyola, and only the Chalcedonians in theory venerate Justinian - in practice, he is not well venerated.
Conversely, in those cases where all of the ancient churches agree that someone is a saint, that tends to be compelling. This is even more the case when the person in question is venerated by the four ancient churches (EO, OO, RC and Assyrian) after the schisms that divided them. The most recent universally accepted saint is St. Isaac the Syrian, an 8th century Assyrian monastic venerated by all of the ancient churches.
