Do you know the difference between a doctrine and a discipline?
If not, you need to!
I have found that until a Catholic understands the distinction, he will be at a great disadvantage when someone challenges him on Church teaching, or when his own doubts creep in.
Here we go....
A doctrine is an unchanging Truth, part of what we call The Deposit of Faith (a.k.a. Sacred Tradition). The Deposit of Faith is the body of truth (faith and morals) that Christ left to His Apostles. The Apostles' successors (Popes and bishops) have preserved and passed this body of truth down through the generations. When Jesus promised His Apostles that the Holy Spirit would come to "lead you to all Truth" (John 16:13), He was talking doctrine.
Doctrine can be better understood over time, and through the centuries the Church has fleshed out its richness (this is called development of doctrine), but its essence does not change. Indeed, the Deposit of Faith can never be contradicted, reversed, added to, or subtracted from. The Holy Spirit sees to that.
Some examples of doctrines: The Ten Commandments; the truth and meaning of human sexuality; the nature of Christ and the Trinity; the Marian doctrines; the basic elements and nature of the seven Sacraments; the male-only priesthood, the Cycle of Redemption.
Hint for thinking of "faith and morals":
Faith = The Creed (what we believe)
Morals = The Ten Commandments (how we live)
A discipline is a rule or regulation which can and often does change. This is the "binding and loosing" authority that the Church received when Jesus said to Peter and the Apostles, "Whatever you bind on earth will be bound in Heaven, and whatever you loose on earth will be loosed in Heaven" (Matt. 16:19, Matt. 18:18). These rules/disciplines can be changed, but when they are in effect, the faithful are bound to them. Why do they exist? To help the faithful in each era become holy. Depending on times and cultures and circumstances, Popes and bishops will bind or loose the faithful according to the needs of the people of God at that time.
Some examples of disciplines: Canon Law; days of fasting and abstinence; Holy Days of Obligation; regulation of religious orders; priestly celibacy; liturgical rubrics (i.e., language of the liturgy, words/prayers/readings for liturgical celebrations, postures and gestures, etc.).
So, when someone says to you, "The Church is not the True Church because it changed its rules on eating meat on Fridays!" you say, "That's a discipline, and it can change!"
And when someone says, "The Church is going to change its teachings on contraception and homosexuality!" you say, "Those are doctrines, and they will never change!"
Any questions? I love questions!