Before the Second Vatican Council, Rahner worked alongside Yves Congar, Henri de Lubac, and Marie-Dominique Chenu, theologians associated with the emerging school of theological thought known as Nouvelle théologie.

Deeply affected by the spirituality of Ignatius of Loyola during the initial phase of his formation (1922–24), he concentrated the next phase of his formation (1924–7) on Catholic scholastic philosophy and the modern German philosophers: he seems to have been particularly interested in Immanuel Kant and two contemporary Thomists, the Belgian Jesuit Joseph Maréchal and the French Jesuit Pierre Rousselot, who were to influence Rahner's understanding of Thomas Aquinas in his later writings.

Rahner was ordained a priest on 26 July 1932, and then made his final year of tertianship, the study and taking of Ignatius' Spiritual Exercises, at St. Andrä in Austria's Lavanttal Valley.

He then returned to the theology faculty at Innsbruck and taught on a variety of topics which later became the essays published in Schriften zur Theologie ([7]): the collection is not a systematic presentation of Rahner's views, but, rather a diverse series of essays on theological matters characterised by his probing, questioning search for truth.

[8] In early 1962, with no prior warning, Rahner's superiors in the Society of Jesus told him that he was under Rome's pre-censorship, which meant that he could not publish or lecture without advance permission.

Rahner's influence at Vatican II was thus widespread, and he was subsequently chosen as one of seven theologians who would develop Lumen gentium, the dogmatic explication of the doctrine of the Church.

[e][10] The council's receptiveness towards other religious traditions may be linked to Rahner's notions of the renovation of the church, God's universal salvific revelation, and his desire to support and encourage the ecumenical movement.

He published several volumes (23 total in English) of collected essays for the Schriften zur Theologie (Theological Investigations), expanded the Kleines theologisches Wörterbuch (Theological Dictionary), co-authored other texts such as Unity of the Churches: An Actual Possibility with Heinrich Fries, and in 1976 he completed the long-promised systematic work, Foundations of Christian Faith.

[h] In addition to his own work, the reference texts that Rahner edited also added significantly to the general impact of his own theological views.

"[18] Written near the end of his life, Rahner's Foundations of Christian Faith (Grundkurs des Glaubens) is the most developed and systematic of his works, most of which were published in the form of essays.

Nonetheless, some theologians and Christian philosophers (e.g., Jürgen Moltmann) have found his teaching to tend strongly in a Modalist direction.

[citation needed] John Hardon defined "transfinalization" as: The view of Christ's presence in the Eucharist that the purpose or finality of the bread and wine is changed by the words of consecration.

[21]Pope Paul VI in the encyclical Mysterium fidei, mentioning the term 'transfinalization' one time, without citing Rahner's name, stated: [...]it is not permissible to[...] discuss the mystery of transubstantiation without mentioning what the Council of Trent had to say about the marvelous conversion of the whole substance [...] as if they involve nothing more than "transignification," or "transfinalization" as they call it...[22] Haldon states that in Mysterium fidei Pope Paul VI "condemned" the notion of "transfinalization", which was one of the "principal errors about the Real Presence that were already current in his day".

[21] The basis for Rahner's theology is that all human beings have a latent ("unthematic") awareness of God in any experiences of limitation in knowledge or freedom as finite subjects.

The book's preface describes the limits of Catholic theology with respect to evolution, further on giving a summary of official church teaching on the theory.

[23] In his writing, Rahner does not simply deal with the origin of man but with his existence and his future, issues that can be of some concern to evolutionary theory.

Accordingly, Rahner doubts the real possibility of a state of pure nature (natura pura), which is human existence without being involved with grace.

"[30] Rahner has been open to the prospect of extraterrestrial intelligence, the idea that cosmic evolution has yielded sentient life forms in other galaxies.

To capture the relation between these aspects of grace, Rahner appropriates the Heideggerian terminology of "thematization:" the objective mediation is the explicit "thematization" of what is always already subjectively proffered – the history of grace's categorical expression without, culminating in the event of Jesus Christ, is the manifestation of what is always already on offer through the supernatural existential, which enters amid a transcendental horizon within.

For this reason, Rahner denies the possibility of a state of pure nature (natura pura, human existence without being-involved with grace), which according to him is a counterfactual.

In fact, it appears there are some limitations of classic Christological formula suggested by the Council of Chalcedon (AD 451) which affirms "one identical Son, our Lord Jesus Christ ... perfect both in his divinity and in his humanity ... [with] two natures without any commingling or change or division or separation ... united in one person.

"[37] Moreover, the Chalcedon formula adopts philosophical concepts such as nature and hypostatic union which are no longer used to explain and interpret religious experiences.

[42] According to Rahner, Christian faith affirms that the cosmos reaches its final fulfillment when it receives the immediate self-communication of its own ground in the spiritual creatures which are its goal and its high point.

"[48] At this point Rahner proposes two possibilities of human salvation, i.e. either as "fulfillment in an absolute sense" which means the establishment of the Kingdom of God on earth, or as "a historical event within history".

In this sense, Rahner sees the incarnation of God as "the unique and highest instance of the actualization of the essence of human reality".

[53] To answer the question of how we find a God-Man in history, Rahner employs a historical approach to Christology by examining the history of the life and death of Jesus of Nazareth and proposes two theses beforehand: 1) Christian faith requires a historical basis; and 2) considering a possibility of significant difference between who the person is and the extent to which that person verbalises or expresses identity, it is possible both to say that "the self-understanding of the pre-resurrection Jesus may not contradict in an historical sense the Christian understanding of his person and his salvific significance", and to state that his self-understanding may not coincide with the content of Christological faith.

[55] There are several historical elements concerning Jesus' identity as a Jew and "radical reformer": his drastic behaviour in solidarity with social and religious outcasts based on his belief in God, his essential preaching "as a call to conversion", his gathering disciples, his hope for conversions of others, his acceptance of death on the cross "as the inevitable consequence of fidelity to his mission".

Inspiration for this idea sometimes comes from the Second Vatican Council's Lumen gentium, which teaches that those "who through no fault of their own do not know the Gospel of Christ or His Church, but who nevertheless seek God with a sincere heart, and moved by grace, try in their actions to do His will as they know it through the dictates of their conscience—those too may achieve eternal salvation.

Non-Christians could have "in [their] basic orientation and fundamental decision", Rahner wrote, "accepted the salvific grace of God, through Christ, although [they] may never have heard of the Christian revelation.

[64] The presence of Christ in other religions operates in and through his Spirit[65] and non-Christians respond to the grace of God through "the unreflexive and 'searching Christology'" (searching "memory" of the absolute saviour) present in the hearts of all persons.