Domenico Beccafumi's Tanaquil from Renaissance Siena: Art for a City. Photograph: National Gallery
Seeing Renaissance Siena: Art for a City at the National Gallery was a revelation, throwing up magnificent artists who have been airbrushed out of art history thanks to Florentine dominance. But it was also slightly discomfiting: I consider myself an open-minded atheist, but - not for the first time - I was left utterly cold by the part of the show devoted to religious art.
It isn't that I can't appreciate the magnificence of the techniques on display in the devotional paintings. Matteo di Giovanni's Assumption of the Virgin altarpiece (1474) is a pious fiesta of angels dancing around the Virgin against a blinding gold background, and Francesco di Giorgio's Saint Dorothy (c.1460) is an elegant vision of the saint and the Christ child. There is nothing technically wrong with these paintings but they fail to stir anything within me.
I can imagine the experience I am missing. If a religious person approached di Giovanni's altarpiece in its original position in Asciano's church, they might see this painting shimmering under dusty rays, glowing with the Virgin's aura of holiness. It would be awe-inspiring in the old-fashioned sense. But to me it's just gold leaf and too many halos.
This has been true of countless museums and galleries I have been round. In the ceiling of the Sistine Chapel I see the wonder of Michelangelo's creativity and genius, admiring his divine technique but not experiencing the divine suffusion those around me seem to.
I do not have this problem with paintings of religious scenes told as narrative, since these do not feel like entirely devotional objects, demanding piety. The Master of the Story of Griselda has almost an entire room devoted to his work, including the cinematic triptych telling the Biblical story of the patient wife Griselda, which has traditionally elegant Sienese figures among fine architecture. The room given over to Domenico Beccafumi is astounding, with his rapid, colourful style illuminating his modern saints, which leavens its religion with wit.
Some of the secular work is especially wonderful. A particular favourite was Luca Signorelli's Two Nude Youths, which is far from the slight Sienese figures - it is fleshy and realistic, curved and much more like Lucien Freud than show star Francesco di Giorgio.
Do you have to be a religiously-inclined person - or even a person of just the religion concerned - to relate to these works? It seems that to fully understand their true purpose, to grasp the reason these paintings are meant to be so special, you have to approach them with God in your heart. As an atheist, these paintings do not speak to me.