The Catholic Church depicts 'Mystic' to be a supernatural act or condition, which a person cannot achieve by one's own effort or force. The cause is an act of God's grace, whereby God reveals a knowledge which cannot be obtained by human ability. On one hand, the revelation may refer to the contemplation of God, or on the other hand, to the vision of Saints, Angels and the rest of created beings (Poulin, p.5ff; p.61).
In chapters 'Visions', 'Linguistic phenomenon' and 'Advice and instructions', Therese's mystic union with God is especially mentioned, e.g., when receiving Holy Communion, in the contemplation of the historical events of the Life and Sufferings of Christ, the Acts of the Apostles, the Revelation and the life of different Saints. To be added are her visions of Heaven and Purgatory. She had no vision of Hell, she only saw the fallen angels in a vision of the 'fallen angels' and the 'Temptation of Christ' (Steiner, Visionen, vol.2, p.134).
Short explanation, to understand the used terminology of the so called 'Last Things', according to the Teachings of the Catholic Church (see also Denzinger-Hünermann; Holböck, Purgatory, p.23ff): the human existence does not end with death, but the elements of body and soul separate. Only the element having awareness and will power, i.e. the soul, will continue to subsist in the intervening time until the 'Last Judgment' (DzH 1000). Immediately after death follows an individual judgment, whereupon the soul will go to Heaven, to Purgatory or to Hell (DzH 857).
The Heaven is open to those souls, who have not committed any sin since their baptism (DzH 1305), to those, who were completely purified on earth or in purgatory or did enough restitution (DzH 1305), or children who died after baptism, yet before being able to use their free will (DzH 1000). These blessed departed will see the nature of God, the Divine Emanations, directly and face to face (DzH 1000).
To Purgatory will go the souls of those people who died in the state of grace - so partaking in the community of the Saints -, but have not yet accomplished sufficient restitution for their sins and the consequential punishments (DzH 1580). The nature of purification is a temporary exclusion from the blissful vision of God, thus a punishment different from those of being condemned (DzH 4657). These 'Poor Souls' cannot redeem themselves, but rely upon the intercession of others, either of the Blessed or the faithful on earth (DzH 1405). The faithful on earth can help them by offering Holy Mass, prayers, giving alms to the poor, penitential exercises and indulgences (Suffragien) (DzH 1753, 1306, 4657).
The souls of those having made a free decision against God, and who die without remorse in the state of mortal sin, go to Hell (DzH 443, 1306). There they will be excluded from the vision of God forever and will suffer eternal punishment (DzH 1002, 1306, 4657). The punishment of Hell is characterized by torment, torture and fire (DzH 2626).
Therese's mystic visions of Heaven happened regularly on the Feast of all Saints. On November 1, 1928, Pastor Naber wrote into his diary: "In the early morning at 6 o'clock, Therese is allowed to look into Heaven. In the first picture she sees the Saviour, surrounded by Mary, Joseph, the Apostles, the 24 elders, the 7 Archangels and a crowd of other angels. In a second picture she sees the Saviour among the virginal souls (author's remark: those are priests, members of religious orders, virgins), and in a third picture the Saviour among the remaining Saints. She recognizes several, whom she has seen in visions or has known herself while living on earth.
All appear as pure, bright beings; besides the Saviour she sees two having a transfigurated body, Mary and Elija. She is so captivated by what she sees, that she longed for death ..." (Naber, p.75f.). The intercession for the Poor Souls was a concern of Therese. She felt great compassion for the suffering Poor Souls, and offered prayers and sufferings for their redemption all the time (Gerlich, p.229 ff.). Often, she could free souls out of Purgatory and experienced, how a respective soul thanked her and was taken up into heaven (Steiner, Visionen, vol. 2, p.145ff.).
According to Christian doctrine, the idea of an atonement sacrifice has its foundation in the Passion of Christ. His substitutional suffering atones for the sins of mankind, and the entirety of his suffering frees all human beings from Original Sin. In view of the entirety of His suffering and His accomplished redemption, the question arises, do sacrificial sufferings of a human being make any sense? The Church affirms this with the argument, that the community of the faithful is one unity, all the offerings are for the benefit of all Christianity and serve the redemption of the whole world (DzH 3363). Therese explains the significance of human atonement sufferings with simple words: "The Saviour is just. For that reason, He has to punish. But He is also kind and wants to help. The committed sin needs to be punished. But if somone else takes the suffering for this, justification is done and the Saviour is free to be merciful" (Gerlich, vol.1, p.300).
How important the redemption of the Poor Souls was for Therese shows the bequest to her brother Ferdinand: "Do not forget the departed. Pray every day for the departed, not only pray, but offer all difficulties coming on to you for the departed. As long as they are alive, we care for them and try to help them. Now, when they need our help, we usually don't think of them anymore. But they are waiting and need our help. They cannot help themselves. However, now, being in the next world, they are closer to us than ever before they could be when alive". And in an urgent manner she added: "Do not keep this, what I have said, for yourself, but tell all the people you ever will meet!".