Question: According to Catholicism, an act of heresy is a mortal sin.

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> "Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same ...
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[Catechism of the Catholic Church #2089](https://www.vatican.va/archive/ENG0015/__P7C.HTM)

In particular, dogmatic statements end with warnings that not accepting the dogma results in falling away from the Catholic faith. For example,

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> Hence if anyone, which God forbid, should dare willfully to deny or to call into doubt that which we have defined, let him know that he has fallen away completely from the divine and Catholic Faith.
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[Munificentissimus Deus #45](https://www.vatican.va/content/pius-xii/en/apost_constitutions/documents/hf_p-xii_apc_19501101_munificentissimus-deus.html)

Why is this the case? Why do matters of doctrine result in loss of salvation?

Answer: 
[Heresy is a species of unbelief](https://isidore.co/aquinas/summa/SS/SS011.html#SSQ11A1THEP1), [which is a sin](https://isidore.co/aquinas/summa/SS/SS010.html#SSQ10A1THEP1) against the theological virtue of faith, [which resides in the intellect](https://isidore.co/aquinas/summa/SS/SS004.html#SSQ4A2THEP1).

St. Thomas Aquinas, discussing whether unbelief is the greatest sin, says ([*Summa Theologica* II-II q. 10 a. 3](https://isidore.co/aquinas/summa/SS/SS010.html#SSQ10A3THEP1) co.):

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> Every sin consists formally in aversion from God, as stated above ([I-II q. 71 a. 6](https://isidore.co/aquinas/summa/FS/FS071.html#FSQ71A6THEP1); [I-II q. 73 a. 3](https://isidore.co/aquinas/summa/FS/FS073.html#FSQ73A3THEP1)). Hence the more a sin severs man from God, the graver it is. Now man is more than ever separated from God by unbelief, because he has not even true knowledge of God: and by false knowledge of God, man does not approach Him, but is severed from Him.
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Commenting on [Hebrews 11:6](https://drbo.org/cgi-bin/d?b=drl&bk=65&ch=11&l=6-#x),

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> But without faith it is impossible to please God. For he that cometh to God must believe that he is: and is a rewarder to them that seek him.
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[St. Thomas writes](https://aquinas.cc/la/en/%7EHeb.C11.L2.n575.2):

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> no one can please God without coming to him: *draw near to God and he will draw near to you* ([Jas 4:8](https://drbo.org/cgi-bin/d?b=drl&bk=66&ch=4&l=8-#x)); *draw near to him and be enlightened* ([Ps 34:6](https://drbo.org/cgi-bin/d?b=drl&bk=21&ch=33&l=6-#x)). But no one draws near to God except by faith, because faith is a light of the intellect. Therefore, no one can please God except by faith. But anyone drawing near by faith must believe the Lord.
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The reason is in the definition of heresy. As you stated

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> "Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same"
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If some truth must be believed with divine and catholic faith, then one who refuses to affirm that truth cannot be said to have divine and catholic faith. If that one does not have divine and catholic faith, then he is spiritually dead, since he is ultimately refusing to believe what God has revealed. Only mortal sin causes spiritual death.

This has to be an obstinate denial, because mortal sin is something you can only do "on purpose," so to speak. If you have not been corrected by anyone in your heresy (if you don't know it's heresy), you have a genuine invincible ignorance, so you don't mortally sin. Once someone gives you an explanation and presents the correct doctrine and shows what the Church teaches, if you go on denying the truth after this, you are sinning.