Objection 1. It seems that fear is not a mortal sin. For, as stated above (I-II, 23, 1), fear is in the irascible faculty which is a part of the sensuality. Now there is none but venial sin in the sensuality, as stated above (I-II, 74, 4). Therefore fear is not a mortal sin.
Objection 2. Further, every mortal sin turns the heart wholly from God. But fear does not this, for a gloss on Judges 7:3, "Whosoever is fearful," etc., says that "a man is fearful when he trembles at the very thought of conflict; yet he is not so wholly terrified at heart, but that he can rally and take courage." Therefore fear is not a mortal sin.
Objection 3. Further, mortal sin is a lapse not only from perfection but also from a precept. But fear does not make one lapse from a precept, but only from perfection; for a gloss on Deuteronomy 20:8, "What man is there that is fearful and fainthearted?" says: "We learn from this that no man can take up the profession of contemplation or spiritual warfare, if he still fears to be despoiled of earthly riches." Therefore fear is not a mortal sin.
On the contrary, For mortal sin alone is the pain of hell due: and yet this is due to the fearful, according to Apocalypse 21:8, "But the fearful and unbelieving and the abominable," etc., "shall have their portion in the pool burning with fire and brimstone which is the second death." Therefore fear is a mortal sin.
I answer that, As stated above (Article 1), fear is a sin through being inordinate, that is to say, through shunning what ought not to be shunned according to reason. Now sometimes this inordinateness of fear is confined to the sensitive appetites, without the accession of the rational appetite's consent: and then it cannot be a mortal, but only a venial sin. But sometimes this inordinateness of fear reaches to the rational appetite which is called the will, which deliberately shuns something against the dictate of reason: and this inordinateness of fear is sometimes a mortal, sometimes a venial sin. For if a man through fear of the danger of death or of any other temporal evil is so disposed as to do what is forbidden, or to omit what is commanded by the Divine law, such fear is a mortal sin: otherwise it is a venial sin.
Reply to Objection 1. This argument considers fear as confined to the sensuality.
Reply to Objection 2. This gloss also can be understood as referring to the fear that is confined within the sensuality. Or better still we may reply that a man is terrified with his whole heart when fear banishes his courage beyond remedy. Now even when fear is a mortal sin, it may happen nevertheless that one is not so wilfully terrified that one cannot be persuaded to put fear aside: thus sometimes a man sins mortally by consenting to concupiscence, and is turned aside from accomplishing what he purposed doing.
Reply to Objection 3. This gloss speaks of the fear that turns man aside from a good that is necessary, not for the fulfilment of a precept, but for the perfection of a counsel. Such like fear is not a mortal sin, but is sometimes venial: and sometimes it is not a sin, for instance when one has a reasonable cause for fear.
Source: Summa Theologica (New Advent)