Patriarch Alexy II, head of the Russian Orthodox Church, leaves Thursday on an international tour that is likely to culminate in a historic meeting with Pope John Paul II. The meeting, which will reportedly take place on June 21 in a suburb of Vienna, Austria, would be the first ever encounter between the heads of the world's two biggest Christian churches and a turning point in their ancient history of mutual suspicion. While the meeting is unlikely to make progress toward unifying the two churches, a move favored by some liberals in both, it could defuse tensions over growing competition between Catholics and the Orthodox in Russia, Ukraine and Belarus. Negotiations for the religious summit have been conducted for months in strict secrecy and church officials both in Moscow and in the Vatican have refused to confirm officially that the meeting will take place. Izvestia quoted a senior cardinal as saying during the pope's recent visit to Poland that a meeting was ""predetermined.