St Sergius of Radonezh

Bartholomew was born in 1314 to the pious boyars Cyril and Maria of Rostov, who later settled in Radonezh. Sent to school at seven, he could not learn to read and prayed day and night for understanding. One day, looking for lost horses, he met a holy elder under an oak who prayed over him and gave him a piece of prosphora. From that day the boy read with ease. After his parents died he went into the forest with his brother Stephen and built a small church dedicated to the Holy Trinity. There, as the monk Sergius, he founded the monastery that became the Holy Trinity-St Sergius Lavra. Humble, laboring with his own hands, and visited by the Mother of God, he became the spiritual father of Russian monasticism. He reconciled princes and sent St Dmitry Donskoy to Kulikovo with his prayers. He reposed in 1392, and his incorrupt relics still rest at the Lavra.

These are Russian patronages. The custom of praying to St Sergius before exams and for children who find study hard comes from his childhood struggle to read; it is strong in Russia but almost unknown in Greek usage, where the Three Hierarchs are the patrons of learning. St Sergius is on Greek and other calendars, but popular devotion is mostly Russian. His dedication to the Holy Trinity inspired St Andrei Rublev's Trinity icon, painted in his honor.