It may seem odd, but that is your problem, as despite your premise that Romes AIM is necessary for assurance of doctrine, for centuries Rome failed to provide its infallible canon, as documented, and thus if you asked your question prior to then it would have been a promise, not a reality.
But again, while the church can speak infallible truth, it is not assuredly infallible according to Rome's formula and assurance that a statement is true is not by this, but truth claims will be established as being from God after the manner of “manifestation of the truth” that established men of God as being such, and writings as being Scripture, long before Rome presumed to be as God, and by such persuasion souls believed the gospel of Christ, (Acts 17:2,11; 28:23; Rm. 15:19, etc.) to the glory of God.
here’s what Christians believe and we have for 2,000 years:
1. Jesus gave HIS authority to TEACH and BAPTIZE to the Church in Matthew 28.
2. The Apostles went all thru the known world preaching the Gospel and building THE CHURCH.
3. The Apostles instructed that the men they ordained, also ordain others to continue this work. Read 2 Timothy 2:1-2.
4. The Holy Spirit moves within THE CHURCH and protects THE CHURCH, he doesn’t work outside the Body of Christ.
5. any time there is a question of doctrine, the Holy Spirit leads THE CHURCH to truth.
6. Christians look to the leaders of THE CHURCH, those men who have their office by “apostolic succession”, to decide doctrinal matters.
7. Hebrews instructs Christians to obey these leaders in 13:17.
8. in the last days, men will become lovers of THEMSELVES, rejecting THE FAITH and following their own way and not God’s way. read 2 Timothy 3:1-9
9.Paul told us to reject anyone else who brings a different Gospel than THE FAITH we received from the Apostles in Galatians 1:6. so when someone teaches BAPTISM is not for the remission of sins, but rather is a test of obedience, LET THEM BE ACCURSED. when someone teaches the Eucharist is NOT THE BODY OF CHRIST, LET THEM BE ACCURSED.
This is what Christians believe.