In 1415 the interests were way more local. As far as I know, the major naval expansion that Portugal undertook that year was the conquest of Ceuta, a city in Northern Africa. The motivation behind that specific conquest was not to establish a trade route all the way to India, it was to control the Mediterranean trade routes around those places, as well as to continue the Reconquista / Holy War against the Moors.

This religious aspect would still be important, along with the trade aspect. At first the Portuguese were trying to get behind the Muslim's dominions, to find an ally to fight them (the mythical Prester John). This was done at the same time as they were conquering strategic places to stop the Barbary pirates attack on Portugal and to spread the Catholic religion. The Portuguese Crown, which controlled the Crusader Order of Avis / Ordem de Cristo (the successor of the Templar Order in Portugal), was the one who financed the martitime expeditions, along with the merchants.

As the Portuguese explored the region, trying to find strategic places to stop the Barbary attacks and allies to help in their fight against the Moors, they ended up trading with people they hadn't met before in Africa and the trade aspect started to get more important (but the religious one would never disappear, these were very religious people we're talking about).

The Portuguese would aim to arrive at India starting with the reign of D. Joao II, in the 1480s.