817 In fact, “in this one and only Church of God from its very beginnings there arose certain rifts, which the Apostle strongly censures as damnable. But in subsequent centuries much more serious dissensions appeared and large communities became separated from full communion with the Catholic Church -** for which, often enough, men of both sides were to blame.**” The ruptures that wound the unity of Christ’s Body - here we must distinguish heresy, apostasy, and schism - do not occur without human sin:
Where there are sins, there are also divisions, schisms, heresies, and disputes. Where there is virtue, however, there also are harmony and unity, from which arise the one heart and one soul of all believers.
It’s a very complicated and sad story but if you want a very simple explanation, East and West had been becoming estranged for a long period of time, communications not being what they are in modern times. In 1054 matters came to a head with not particularly good behaviour by the then Patriarch of Constantinople and the Papal envoy, a Cardinal. Even after this, a full break in communion was not made for a couple of more centuries.
About the Orthodox
1399 The Eastern churches that are not in full communion with the Catholic Church celebrate the Eucharist with great love. “These Churches, although separated from us, yet possess true sacraments, above all - by apostolic succession - the priesthood and the Eucharist, whereby they are still joined to us in closest intimacy.” A certain communion in sacris, and so in the Eucharist, “given suitable circumstances and the approval of Church authority, is not merely possible but is encouraged.”