We cannot say that one is the “true” church, for the Church is the community of all baptized Christians.
The Orthodox Church is *almost *in full communion with the Catholic Church. Learn more about the “East-West schism” or “Great Schism” of 1054. In short, it was a sad fight between brothers that ended with a mutual excommunication, revoked only by both sides on 1965.
According to the Catechism:
with the Orthodox Churches, this communion is so profound "that it lacks little to attain the fullness that would permit a common celebration of the Lord’s Eucharist.
What you must take into consideration, however, is this fundamental teaching:
Christ “established here on earth” only one Church and instituted it as a “visible and spiritual community” …] This Church, constituted and organised in this world as a society, subsists in the Catholic Church, governed by the successor of Peter and the Bishops in communion with him”
You can learn more here and elsewhere.
Overall, though, I think that the major point is that it was Christ’s direct prayer to the Father that we would become “one flock with one shepherd”, and this became clear with the primacy of Peter between the Apostles, acknowledged even by St. Paul. It will only be a matter of time before there is full reconciliation between the Catholic and the Orthodox Church under one shepherd