To deny Baptism of Desire is heresy. For those that reject this de fide teaching of the Church, you place your soul in grave peril.
Is the de fide teaching limited to actual catechumens and martyrs?
The
de fide teaching only applies to explicit Baptism of Desire. When the Holy Office explained this teaching in the 1949 letter,
Suprema Haec Sacra, it referenced the Council of Trent as teaching explicit Baptism of Desire. It is for this reason that anyone who denies Baptism of Desire professes a heresy against the Faith.
Regarding implicit Baptism of Desire, the Holy Office corrects the Saint Benedict Center for this error against the Faith, but does not accuse them of heresy. The reason is that the Saint Benedict Center in its publication reviewed by the Holy Office had not denied Baptism of Desire itself, rather the Church's teaching on implicit Baptism of Desire.
Msgr. Fenton explains:
The most important error contained in that article was a denial of the possibility of salvation for any man who had only an implicit desire to enter the Catholic Church. There was likewise bad teaching on the requisites for justification, as distinguished from the requisites for salvation. The first of these faults has been indicated in a previous issue of The American Ecclesiastical Review.[12]
The teaching on implicit Baptism of Desire must be believed as it is both authoritative teaching of the Pope's ordinary magisterium, (
Mystici Corporis), and is also certain doctrine as this teaching is taught by the consensus of the theologians.
If a Catholic denies implicit Baptism of Desire as explained by the Holy Office letter, he would objectively commit a mortal sin, but would not be outside the Church.