By Rome's standards, the Church of Sant'Ignazio di Loyola seems like it isn't very old at all – only consecrated in 1722 – but that's because prior to 1650, it was a private church.
Saint Ignatius of Loyola was the founder of the Society of Jesus – better known as Jesuits – and the original church on this site was built entirely by Jesuit labor in the 1560s on the foundation of an earlier building. That church, built as the private chapel for the Collegio Romano (the first Jesuit university), was expanded slightly in 1580, but by the early 1600s it was already too small for the number of students at the college. Construction on the current church was started in 1626, a mere four years after Saint Ignatius of Loyola was canonized, and it opened to the public in 1650.
The interior reflects the church's Baroque style with heavy ornamentation. There is gold decoration everywhere, enormous frescoes, and Jesuit iconography and stories depicted throughout. There's even an enormous trompe l'oeil ceiling, with an indication on the floor where to stand to get the best view.