Please answer a question about the following article about Greeks:

The relationship between ethnic Greek identity and Greek Orthodox religion continued after the creation of the Modern Greek state in 1830. According to the second article of the first Greek constitution of 1822, a Greek was defined as any Christian resident of the Kingdom of Greece, a clause removed by 1840. A century later, when the Treaty of Lausanne was signed between Greece and Turkey in 1923, the two countries agreed to use religion as the determinant for ethnic identity for the purposes of population exchange, although most of the Greeks displaced (over a million of the total 1.5 million) had already been driven out by the time the agreement was signed.[note 1] The Greek genocide, in particular the harsh removal of Pontian Greeks from the southern shore area of the Black Sea, contemporaneous with and following the failed Greek Asia Minor Campaign, was part of this process of Turkification of the Ottoman Empire and the placement of its economy and trade, then largely in Greek hands under ethnic Turkish control.

What factor was used to expel or include citizens in the Lausanne Treaty ?
agreed to use religion as the determinant for ethnic identity for the purposes of population exchange