This equating of Constantine with Catholicism is an error because the basic structure of the Church and the initial creeds were already being formulated before Constantine and the faith that was embraced was decided upon without the emperors, even though they did their darnedest to try to manipulate and maneuver the debate. Ultimately it was God who triumphed because it was the bishops and not the emperors who determined the shape of the Church.
With Constantine there was a second icon that represents Christ, even with his shortcomings, the Roman Emperor really did represent Christ’s reign on earth. The view in the Eastern Empire was to accept the Emperor as a religious leader and this is often called caesaropapism (one person with both spiritual and civil authority). In the West however, especially with the figure of the pope, whose spiritual authority was exercised in a more immediate way, there was less of a tendency to look for the Emperor for guidance, but rather to papal authority. This is not to say that the pope’s authority was rejected in the East because the pope’s authority existed there (when disputes arose) and his opinion was sought by Eastern bishops. The Emperor was more of a focal point for the whole life of the empire, including Church life, in the later Middle Ages. It will take the Church a while to adjust to the challenge of the Emperor’s interference. By the end of the century, the Emperor Theodosius (379-395) basically "outlawed" paganism, forbade sacrifice and outlawed different heresies. The challenge of caesaropapism would be one the Church would struggle with throughout the Middle Ages, but she remained the instrument Christ founded for the salvation of mankind.
Jesus’s promise of the Holy Spirit given in the Gospel of John (chapters 14-16) and in his foundation of the Church, when he told Peter and the Apostles, “And I tell you, you are Peter, and on this rock I will build my church and the gates of hades [i.e., powers of death or hell] shall not prevail against it” (Mt. 16: 18). The Holy Spirit was present in every chapter of the Book of Acts from Pentecost to the end of the narrative on St. Paul’s work of founding churches, appointing bishops and presbyters as the apostle to the Gentiles. Paul himself referred to the Church as the “pillar and foundation of truth” (1 Tim 3: 15) because of the work of the Holy Spirit within her. The Holy Spirit is the sanctifier, who was sent by the Father and Son to complete the work of the Son. He makes "holy." As Pope John Paul II has written, "Having accomplished the work that the Father had entrusted to the Son on earth (John 17:4), on the day of Pentecost the Holy Spirit was sent to sanctify the Church for ever, so that believers might have access to the Father through Christ in one Spirit" (Eph 2: 18).
Are we to now assume that the Word was wrong, that the Holy Spirit would be departing from the Church for more than a millennium, despite the promise of Jesus? At a time when the canon of the Bible had not yet defined, are we ready to bid farewell to the Spirit for an extended period of time? As the Catechism of the Catholic Church (CCC) observes:
The mission of the Christ and the Holy Spirit is brought to completion in the Church, which the Body of Christ and the Temple of the Holy Spirit. This joint mission henceforth brings Christ’s faithful to share in his communion with the Father in the Holy Spirit. The Spirit prepares men and goes out to them with his grace, in order to draw them to Christ. . . .Thus the Church’s mission is not an addition to that of Christ and the Holy Spirit, but is its sacrament (CCC 737-738).
We should also remember that Christ himself is the bridegroom of the Church (Ephesians 5: 24; 2 Corinthians 11: 2). The Church is pictured in Revelation 21: 9 as the bride of the Lamb, the new Jerusalem or as St. Paul referred to it, the “new Israel.” She will as Christ said have both wheat and chaff within her until judgment day, but she will never cease to have the Holy Spirit at work within her. Pope John Paul II in his encyclical letter, Mission of the Redeemer wrote:
. . . .the universal activity of the Spirit is not to be separated from his particular activity within the body of Christ, which is the Church. Indeed, it is always the Spirit who is at work, both when he gives life to the Church and impels her to proclaim Christ, and when he implants and develops his gifts in all individuals and peoples, guiding the Church to discover these gifts, to foster them and to receive them through dialogue. Every form of the Spirit's presence is to be welcomed with respect and gratitude, but the discernment of this presence is the responsibility of the Church, to which Christ gave his Spirit in order to guide her into all the truth (cf. Jn 16:13).
From the beginning of creation until now the Spirit is working and especially today within the Church Christ founded as the instrument for our salvation. Constantine was certainly not the role model for civil leaders in every respect, nor was he the beginning of an apostasy some hypothesize for the Church founded by Christ, the Catholic Church. Reading the works of the Early Church Fathers is an antidote against the view that the Church was in apostasy. As John Henry Newman put it, "to be deep in history is to cease to be Protestant" (An Essay on the Development of Christian Doctrine).