"Extra Ecclesiam Nulla Salus" is the doctrine at least as old as pope Boniface VIII in its precise idiomatic form. It is, of cours,e scripturally resting on John 3:5 and similar verses elsewhere.
Vatican II confirmed it.
The boundary of the Church is determined by valid baptism of her members, as the unbaptized cannot ordinarily be saved. When a baptized Christian is saved and goes to heaven he does so on the merits of his baptism and his Christian life. He goes to heaven, therefore, as Catholic. It is true that often better examples of Christian life are found in the Orthodox Church, where we see very commendable adherence to the Church life. As I like to repeat, there are probably more Catholics who call themselves Orthodox that who call themeselves Catholic.
Likewise the Protestants who adhere to the Protestant communities of faith due to their cultural background and do not make a career out of attacking the Church, stand a good chance of dying Catholic and be pardoned for not partaking of the true sacraments of the Church in their lifetime.
The danger is great for those who are reasonably well informed of the Church's teachings, but persist with their "protesting".
Then it may please you to know that I don't "attack" the Church. I simply point out the egregiously wrong doctrine and theology the Church purports such as:
That being said, I will go to heaven simply because Christ has paid for my sins with His shed blood, He helped me to see my sinful behavior, I have repented of my wretchedness, and I trust Him to keep me in His care and help me to bear the fruit that He would like me to bear. I simply trust His promises.