St Faustina’s revelations are so-called “private” revelations.
The Catechism of the Catholic Church says:
There will be no further Revelation
66 "The Christian economy, therefore, since it is the new and definitive Covenant, will never pass away; and no new public revelation is to be expected before the glorious manifestation of our Lord Jesus Christ."28 Yet even if Revelation is already complete, it has not been made completely explicit; it remains for Christian faith gradually to grasp its full significance over the course of the centuries.
67 Throughout the ages, there have been so-called “private” revelations, some of which have been recognized by the authority of the Church. They do not belong, however, to the deposit of faith. It is not their role to improve or complete Christ’s definitive Revelation, but to help live more fully by it in a certain period of history. Guided by the Magisterium of the Church, the sensus fidelium knows how to discern and welcome in these revelations whatever constitutes an authentic call of Christ or his saints to the Church.
[INDENT]Christian faith cannot accept “revelations” that claim to surpass or correct the Revelation of which Christ is the fulfilment, as is the case in certain nonChristian religions and also in certain recent sects which base themselves on such “revelations”. (source)[/INDENT]