Giertz stated that his father eventually became Christian after attending the Sunday services that were obligatory in order for the teen-aged Bo and his siblings to be eligible for confirmation; although Giertz was now formally enrolled in the church, he remained an atheist, read widely from his father's library of atheist literature and argued with the priest in favour of evolutionary biology.

Perceiving a conflict between his atheism and his latent moral sense, and concerned at the immoral behavior he had observed among some of his atheist peers, he became involved with a Christian student association.

After attending some lectures by Natanael Beskow, a non-ordained pacifist preacher who led Förbundet för kristet samhällsliv ("the Swedish association for Christian social life"), Giertz became convinced of the existence of God and the historicity of Jesus.

During a semester abroad to study Etruscan archaeology in Italy with Axel Boëthius, an audience with Swedish Queen Consort Victoria left a deep impression upon him.

Born in Germany, the queen had moved to southern Italy, establishing a residence on the resort island of Capri, from where she would visit Sweden in the summers.

Returning to his studies in Uppsala, Bo Giertz was mentored by New Testament exegetics professor Anton Fridrichsen; he later declared that all his books had been written in an endeavour to disseminate what Fridrichsen had taught him, ideas which he believed to be in marked contrast, for instance, to the efforts of German theologian Rudolf Bultmann to demythologize Christianity of accreted concepts of angels, demons, miracles, heaven and hell — all of which Bultmann considered as significant barriers to people's understanding and accepting the inner message of Jesus.

After completing seminary training, Giertz spent three years as a travelling consultant for the Lutheran church's high school student association.

It is not as surprising as it might seem that Rosenius's influence was still felt; he had been a looming presence, for instance, in the household of artist David Wallin as he grew up in the same parish a generation earlier.

Giertz, too, fell under his posthumous sway, to the extent that the Latin phrase Verbum crucis Dei virtus ("The message of the cross is the power of God", 1 Cor.

During this time Giertz began to take seriously Schartauanism, a form of Pietism that had developed in western Sweden — teachings which had been greatly influenced by the works of Henric Schartau — and became inspired by Bokenäs vicar Gösta Nelson.

By now he had come to feel that he was falling short of God's moral dictates; whereas he had previously thought forgiveness of sins grounded in faith alone to have been a "hopelessly naïve" position, he now came to embrace this doctrine of the western Pietists and to take solace in it.

Here he authored many theological works including Kyrkofromhet ("Ecclesiastical piety", which was published with Kristi kyrka in 1939); Den stora lögnen och den stora sanningen ("The great lie and the great truth", 1945); Kampen om människan ("The Battle For Man", 1947); Stengrunden (“The Hammer of God”, 1941);[2] and Tron allena (“Faith Alone”, 1943).

Giertz supported the military campaign, sending money and materials and, after the Moscow Peace Treaty had ceded one-eleventh of Finland's territory to the Soviet Union, receiving Finnish refugees.

His published works from this time of his life included Riddarna på Rhodos ("The Knights of Rhodes", 1972), a novel exploring the theology of the cross amidst the siege of Rhodes in 1522[6] and a two-volume devotional work, Att tro på Kristus and Att leva med Kristus, published in English as “To Live With Christ.”[7] He also retranslated the New Testament into Swedish, accompanied by 12 volumes of commentary (1976-1982).