To study the history of the early Church is to experience the history of the gradual articulation of her identity. First, there was the controversy over the admission of the gentiles. Then came the battle with the Gnostics over the primacy of love over knowledge. Eventually Marcionites and Valentinians dropped away—they tried to differentiate the God of Jesus and the God of the Old Testament, creation and redemption, personal religion and the public, institutional life of the Church. Of course too there were the Christological controversies surrounding Docetism, Monarchianism, and Arianism, in which the Holy Spirit guided the Christian understanding of God as Trinitarian.
Because of the historical reality of the Resurrection, the Catholic faith came under the discipline and guidance of Apostolic tradition and authority. In the early Church, obedience to the eyewitness of those whose experience authorized them to set the tradition was of overriding significance. The truth was what they said it was—they were the authoritative witnesses to the whole reality. It was not a new doctrine up for debate, but a teaching which had to be received. The New Testament brims with concern for unity of faith and life based on reception of the Apostolic tradition.
Yet Catholicism was open to and had the wherewithal to assimilate people of different experiences, absorbing what was greatest in their spiritual cultures. This was because the Church early on saw that she had a universal mission.
Nevertheless, Catholicism is not a syncretistic religion, but one always seeking to bring forth something new as she learns from interactions with every culture and religion. Because it is Catholic, it does not wish to overlook anything in other traditions which is good and touched by grace. (Cf. Luke 9:50) It enters into cultures and seeks to preach the Good News to all peoples through their own language and cultural forms.
Yet we note that in this openness the Church discriminates what it assimilates in accord with its own identity. The student of Church history reads of how the Church assimilated Roman law, Barbarian feasts and mythologies, and Arabic philosophy--but transformed them. It was the Church's fusing energy that led it into dialogue with Hellenistic thought. St. Augustine and Pseudo-Dionysius absorbed Neo-Platonic spirituality, and fashioned a Christian understanding of mysticism. St. Thomas Aquinas engaged Aristotelian philosophy, and developed a synthesis of theology which remains a dominant source of spiritual and theological insight and practice.
The bottom line is that synthesizing and syncretism are fundamentally dissimilar. A syncretistic religion has no identity of its own, whereas a synthetic religion has a clear identity. What the Church absorbs, it transforms, and enhances. Catholicism never puts its own identity and self-understanding in question or regards herself as on par with other traditions, nor understands herself as open to absorption into something higher; she sees herself as that which can absorb the best in other traditions.
At Vatican II the Church recommitted itself to learning from all that is good in other religions, notably the great religions of the East. Perhaps new syntheses will emerge, as the New Evangelization, while recognizing the distinct value of other traditions, uses the culture's own symbolic terminologies to convey the Good News of Jesus Christ, for she possesses a distinctive understanding of the human situation and of how it can be healed, which enables it to discriminate the truth or value of other ideas and practices, and select from them. That which guards the Church's identity is commitment to the risen Christ as the definitive Savior of the world, as He is made known to us through Apostolic witness, Catholic doctrine, and the sacramental life. Lose this, as many seem to be doing today, and all that remains is maudlin syncretism.