Josue, as a former Catholic who had an adult conversion to Evangelical Christianity, and whose whole family is still Roman Catholic, I think I can give you a few pieces of advice. First, there are some good books to get a hold of just to get clarity on what are the doctrinal differences between Roman Catholicism and Evangelical Protestantism.
It is also important to know that for the last 25 years there has been a very positive and, I think, theologically sound movement in the US amongst bible-believing Catholics and Protestants called “Evangelicals and Catholics Together” or ECT. This movement was started by the late Chuck Colson (a great apologist, true saint of the church, and dear colleague of Ravi Z.), and his friend Fr. Richard John Neuhaus. Both are dead now, but the movement lives on and, I think, is bearing much fruit. Here is the most recent book by men and women working out of that ecumenical project:
The good thing about interacting with these authors and theologians is that they are all committed Christians, i.e. committed to a “mere Christianity”, while also holding to their own theological traditions. Therefore, where there is actual and genuine agreement, it is embraced, but where there is still serious intellectual and dogmatic disagreement, those disagreements are not whitewashed in favor of some kind of superficial unity.
Another resource to recommend is the magazine “Touchstone” put out by the fellowship of St. James. This is a great resource, edited and authored by Roman Catholics, Evangelical Protestants, and Easter Orthodox theologians, professors, and writers. Again, ecumenical, but without being naive about the serious nature of our differences.
Since I have led classes on this issue in particular, and because it is very personal to me as well (again, my whole family is Catholic) there is much I could say. I’ll close out this post with a few macro-level thoughts, but we can continue to interact on this if you like.
The main area of disagreement between Catholics and Evangelicals has been and will continue to be the issue of authority. On the historical Protestant view, the doctrine of Sola Scriptura, or scripture as the final arbiter of truth on all issues regarding the Christian faith, fundamentally distinguishes Protestant denominations from Roman Catholicism. Roman Catholicism has two additional sources of authority: the teaching magisterium with the Pope as its infallible head (when the Pope makes infallible pronouncements), and the sacred oral traditions of the church. Those other authorities are ultimately the source of many of the additional Catholic teachings, some of which are not found in your Bible, that you might encounter in your conversations with Catholics.
This is a critical time for both the Roman Catholic Church and, as you know, evangelical churches. The Catholic Church is in a major crisis, not only due to horrific priestly abuse crimes, but also due to changes being made to the catechism at the highest levels of religious authority (i.e. the Pope and his consortium of bishops). This is really a time of turmoil in the Catholic church, and I have many friends and family members concerned about what is going on. It is a good time to encourage faithful Catholics that even when there is trouble in the Church, they still have the Gospel of Jesus Christ to hang on to. So, I think you are doing the right thing in not necessarily challenging some of these extra-biblical Catholic traditions, but redirecting them back to the main truths found in the Scriptures; and the main Truth that Jesus died for our sins. That is something that cannot be lost, no matter who is sitting in Rome on St. Peter’s chair, or how many wolves have crept into the sheep fold.
Hope this helps.