Since the Sacraments were all instituted by Christ, they belong to the Church He founded. One Lord, one Faith, one baptism. Those baptized Christians who reject His One, Holy, Catholic and Apostolic Church are the ones who are in effect rejecting Catholics as "brethren".
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.”324
But it does lack the fullness (which the catechism also states). The existence of the gulf rather than its degree is the material point, ecumaniacal delusions not withstanding.
That is simply presumptuous, as it presumes the church of Rome is the NT church, but which it stands in contradiction to, and that the act of baptism itself makes one born again and formally justified on the basis of their own personal holiness, but which is not Scriptural. (Acts 15:7-9; Rm. 1:1-7ff)
On the contrary, it's the CCC that teaches that those who reject the RCC and will not submit to it's authority are not saved. It's the Catholic church which labels people heretics and ex-communicates them, which means that Catholics are rejecting Christians as brethren.
Luther didn't leave the Catholic church. It ex-communicated him.