Ask a Catholic

[Randy Carson]

Why does doctrine change if the pope is a direct infallible line to god?

A great question! Thank you.

First, infallibility does NOT mean that the pope has all the answers. He can be just as wrong as anyone else on a particular subject. Additionally, he is not impeccable; he commits sins just like the rest of us. HOWEVER, he is prevented from formally teaching error in matters of faith and morals, and that is what we refer to as infallibility. 

As for doctrine changing, the answer is fairly simple: doctrine has NEVER changed in the 2,000 year history of the Church. It can and does develop in the sense that today we see further than the believers of the past because our theology has had more time to mature. In this sense, our theologians, like the modern-day scientists, are standing on the shoulders of giants.

These definitions might help:

Dogma
A dogma is 1) a divinely revealed truth which 2) has been proclaimed as such by the infallible teaching authority of the Church. 
1) A dogma must be found explicitly or implicitly in the deposit of faith given to the Church by Jesus in Sacred Scripture and/or Sacred Tradition.
2) A dogma must be infallibly taught by the Church as divinely revealed. 
All dogmas are infallibly taught as divinely revealed truths, but not all infallibly defined truths are taught as divinely revealed. In the latter case, the infallibly taught truth would not be considered a dogma. 

Doctrine
A doctrine is a way of understanding divine revelation and which is taught authoritatively by the Magisterium of the Church. Doctrine may be defined either infallibly or non-infallibly. All dogmas are doctrines, but not all doctrines are dogmas.

Discipline
A discipline is a rule of the Church which requires obedience due to the authority given to the Church by Jesus. Disciplines may be changed during the course of history based upon the needs of the Church. Examples of Church disciplines include fasting during Lent and the celibacy of priests in the Latin rite.

Devotion
A devotion is a religious exercise or practice other than the regular corporate worship of a congregation. Devotions directed toward God are rightly called worship while devotions directed toward saints are classified as veneration.