Thanks for the question.
With some study and research, I believe you will find the answers you are looking for with respect to the faith of your Grandmother. (Who is probably praying for you from Heaven—because our grandmothers are saints!) The "certitude" for which you "know without a doubt" that the Catholic Faith is true, can be yours when you earnestly take this journey. I would start with a single issue and read everything you can on it, both Catholic and critics, but especially the Early Church Fathers.
You must delve deeper into history to see the unchanging faith since the time of Christ himself (pre-Protestant Reformation) and discover the truth about things like the Mass, the Holy Eucharist, Marian Devotion, Salvation — (Justification and Sanctification).
The Catholic Church makes bold claims that can be researched and verified, but you must do the work. Here is the fundamental claim that you must examine:
Jesus Christ, the Son of God, Second Person of the Most Holy Trinity, came to establish a Church, built on Apostles, who He commissioned to advance His Gospel, for the salvation of souls.
The marks of this Church are: One, Holy, Catholic, and Apostolic. We contend that Christ did indeed establish one Church, not simply an invisible collection of spiritual people who claim to have been saved by Christ, but a visible Church, one that continues generation after generation to carry forward that authority which He imparted to them. While there are many Christian "churches" or denominations in the world, they are all but splinters of the One Visible Church Christ established; we are a broken kingdom, which is a scandal to the world, and an opportunity for Satan to exploit. We should therefore do all we can to fix this great sin of division — and your researching this issue is a great start.
Just to give you something to begin to mull over, let's take the issue of Sola Scriptura. The Protestants contend that the Bible alone is the sole (infallible) authority in matters of faith. They say that God intended for the Bible to be the sole authority.
- Did Jesus Christ ever write a single book or letter that is held by His followers?
- Did He tell His followers to write down anything?
- Did He designate who should write?
- Were the 12 Apostles considered scribes for this purpose?
- Who was designated to be the scribes?
- When were they supposed to write?
- When were these writings to be codified into a collection that would be disseminated to the entire Church?
- Who would ensure their authenticity?
- What language should they be written in?
- Why would God choose writings to be the only infallible source of authority when, for most of history of the world, illiteracy and lack of access to written resources was the norm?
- Are translations of the original writings to be considered "infallible" as well?
In a post printing press world it is hard to fully appreciate or understand the anachronistic implications of a written-only authority structure theory. Consider the difficulties with the US Constitution and how we need a Supreme Court to interpret it —
- Would God have even less foresight?
- Do you see the difficulty of maintaining the sole authority of Scripture without an authority outside of Scripture to verify the canon, authenticity, etc.?
None of these questions can be answered by Scripture alone, which is supposedly the only authority for answering these questions. (A dead end of circular reasoning)
For most of history it was understood that Christ established a Church, for which there are many explicit references in Scripture (i.e., Matthew 16:17-20) and that Church would have his authority (cf. also Matthew 18:15-18, Matthew 28:18). You don't need an inspired Bible to prove this point, history itself is an adequate witness of the establishment of a Church acting in the name of Christ, with an authority structure, councils, order, sacraments, rituals, customs, prayers, works and doctrine. And it also demonstrates that points of doctrine were often in contention but that an Authoritative Church would rule out false doctrines and reaffirm what was considered orthodox Christianity. So you will come to see that Authority rested with the Church, and the Apostles in particular.
They handed that authority on in subsequent generations of bishops, and the Pope is the successor to Peter, the first of the Apostles, the one charged with holding the brethren together and having the supreme authority (cf. Matthew 16:17ff; Luke 22:31; John 21:15ff) That doesn't mean you can't have a bad bishop, priest or even Pope—after all Jesus had Judas, but there are limits to what God allows for the Church and world, for He, ultimately, is in charge. But we often get the leaders we deserve, so if you have seen scandal, that's because God let it happen to show us how far we, as a people, have fallen.
So, I encourage you to go forward and let the Holy Ghost lead your path.