According to the OED, syncretism is
the system or principles of a school founded in the 17th century by George Calixtus, who aimed at harmonizing the sects of Protestants and ultimately all Christian bodies: see Calixtin n. 2
Calixtin n. 2: "George Calixtus (1586–1656), a Lutheran divine and professor at the University of Helmstedt, Brunswick, noted for his moderate and conciliatory views and writings on controversial points"
Although the word "syncretism" was not explicitly used, one of the principal reasons for the convocation of the Council of the Trent by Pope Paul III on 22 May 1542 was to clarify Christian doctrine against Lutheran corruptions of it and thereby foster unity in true doctrine:
[…] since neither Christian and Catholic truth, nor our own dignity nor that of the Apostolic See would permit us to yield in this, we chose rather to command that it be proposed openly that a council be held as soon as possible […] [Because] the truth of whose faith and whose religion are now so violently assailed both from within and without[, we desire to] […] bring speedily and happily to the desired result whatever things pertain to the purity and truth of the Christian religion, to the restoration of what is good and the correction of bad morals, to the peace, unity and harmony of Christians among themselves, of the princes as well as of the people, and whatever is necessary to repulse those attacks of barbarians and infidels whereby they seek the overthrow of all Christendom.
The decree concerning the opening of the council (13 December 1545, the first session, celebrated under Paul III) also implied that "peace and unity" result from "the extirpation of heresies" (i.e., by not syncretically compromising on doctrinal truths and principles):
Does it please you, for the praise and glory of the holy and undivided Trinity, Father, Son, and Holy Ghost, for the advance and exaltation of the Christian faith and religion, for the extirpation of heresies, for the peace and unity of the Church, for the reform of the clergy and Christian people, for the suppression and destruction of the enemies of the Christian name, to decree and declare that the holy and general Council of Trent begins and has begun?
They answered: It pleases us.
quotes from: Schroeder, O.P.'s Canons and Decrees of the Council of Trent
cf. also Jedin's history of the Council of Trent
For a good overview of the Catholic doctrine of unity in the true faith, see Pope Leo XIII's encyclical Satis cognitum: On the Unity of the Church.