Is this a truly authoritative statement?

[Genesis315]

Per the OP, in the address Vico posted above, Pope St. John Paul II affirmed the very passage you quote, as well as two other instances when the Church definitively declared this truth.
St. John Paul II:

Since Christ brings about salvation through his Mystical Body, which is the Church, the way of salvation is connected essentially with the Church. The axiom extra ecclesiam nulla salus"–“outside the Church there is no salvation”–stated by St. Cyprian (Epist. 73, 21; PL 1123 AB), belongs to the Christian tradition. It was included in the Fourth Lateran Council (DS 802), in the Bull Unam Sanctam of Boniface VIII (DS 870) and the Council of Florence (Decretum pro Jacobitis, DS 1351).

vatican.va/holy_father/john_paul_ii/audiences/alpha/data/aud19950531en.html

The DS citations in Latin can be found here (that site also has it in French, but sadly not English):
catho.org/9.php?d=g1

 

[Elaine13]

By the authority of God’s Holy Word, we can be assured that the op statement is not infallible. A statement like that reminds me a little of the conflict over circumcision in Acts 15, where there were those who insisted that in order to be saved, one must be circumcised. Peter settled the dispute nicely with just a few words which also apply to us here:
“Now the apostles and elders came together to consider this matter. And when there had been much dispute, Peter rose up and said to them: “Men and brethren, you know that a good while ago God chose among us, that by my mouth the Gentiles should hear the word of the gospel and believe. So God, who knows the heart, acknowledged them by giving them the Holy Spirit, just as He did to us, and made no distinction between us and them, purifying their hearts by faith.” (v.6-9)

Then Peter admonished them by saying: “Now therefore, why do you test God by putting a yoke on the neck of the disciples which neither our fathers nor we were able to bear? But we believe that through the grace of the Lord Jesus Christ we shall be saved in the same manner as they.” (v. 10, 11)

Furthermore, Peter also says in Acts 2:38, “Then Peter said to them, “Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit.”

Our Saviour Jesus Christ promised us: “I am the door: by me if any man enter in, he shall be saved, and shall go in and out, and find pasture.” John 10:9

Here’s what Paul also tells us plainly in Romans 10: 8-10
“But what does it say? “The word is near you, in your mouth and in your heart” (that is, the word of faith which we preach): that if you confess with your mouth the Lord Jesus and believe in your heart that God has raised Him from the dead, you will be saved. For with the heart one believes unto righteousness, and with the mouth confession is made unto salvation.

I could go on and on, but it is clear to me that nowhere do we find in scripture that “those not living within the Catholic Church, not only pagans, but also Jews and heretics and schismatics cannot become participants in eternal life, but will depart 'into everlasting fire which was prepared for the devil and his angels”.

 

[D0UBTFIRE]

Pope Eugene IV, Papal bull 1441 Cantate Domino

“[the Church] firmly believes, professes, and proclaims that those not living within the Catholic Church, not only pagans, but also Jews and heretics and schismatics cannot become participants in eternal life, but will depart ‘into everlasting fire which was prepared for the devil and his angels’ [Matt. 25:41], unless before the end of life the same have been added to the flock; and that the unity of the ecclesiastical body is so strong that only to those remaining in it are the sacraments of the Church of benefit for salvation, and do fastings, almsgiving, and other functions of piety and exercises of Christian service produce eternal reward, and that no one, whatever almsgiving he has practiced, even if he has shed blood for the name of Christ, can be saved, unless he has remained in the bosom and unity of the Catholic Church.”

And from the Catechism:

1258 The Church has always held the firm conviction that those who suffer death for the sake of the faith without having received Baptism are baptized by their death for and with Christ. **This Baptism of blood, like the desire for Baptism, brings about the fruits of Baptism without being a sacrament. **

These statements seem to contradict eachother.

 

[Vico]

And from the Catechism:

1258 The Church has always held the firm conviction that those who suffer death for the sake of the faith without having received Baptism are baptized by their death for and with Christ. **This Baptism of blood, like the desire for Baptism, brings about the fruits of Baptism without being a sacrament. **

These statements seem to contradict eachother.

Re: “… and that no one, whatever almsgiving he has practiced, even if he has shed blood for the name of Christ**,** can be saved, unless he has remained in the bosom and unity of the Catholic Church.”

No conflict: Those that* were* Catholic are the subject. Those that receive baptism of blood, die without baptism of water, therefore they were never in the bosom and unity of the Catholic Church while living.