The Church of the Holy Cross located on Akhtamar Island in Van Lake.
The church was built between 915 and 921 AD by Architect Bishop Manuel, under the supervision of Gagik I Ardzruni, an Armenian King who reigned over the Kingdom of Vaspurakan as a vassal under the Abbasids (see more at Wikipedia). The church is in the tentative list of UNESCO World Heritage Site.
The Akhtamar Island is 13 km from Gevas boat pier. Gevas is at south coast of Van Lake, in the middle of Tatvan (90 km) and Van (45 km). The closest train station is Tatvan Gar Station.
Take bus from Tatvan main bus station (next to train station) to Gevas or Van. Tell bus driver that you’d like to get off at Akhtamar Boat Pier, which is 9 km before Gevas. There are boat services to Akhtamar Island from pier (13 km).
See direction map from station to Akhtamar Church below in Google Maps:
You can wait until boat is full, or pay for the all seats not to wait.
Van (45 km from pier) has flights to Istanbul, if you prefer faster way for return.
How to get to Akhtamar Church by train from Turkish cities?