The obvious answer is “Because Christ said so.” But how do we come to this conclusion?
In John 14:1-12, Jesus tells doubting Thomas “I AM the Way, the Truth, and the Life…” Not “a”, the. How many times have you heard someone say something like “There are many roads and they all lead to the same place”? In fact, Up With People used to sing a song that said “There are many roads to go, and they go by many names. They don’t all go the same way, but they get there just the same. And I have a feeling that we’ll meet some day where the roads come together up the way.” It’s a very nice sentiment. And absolutely wrong.
Jesus wasn’t speaking a parable, he was answering a question “Master how can we know the way?” And Jesus answered “I am the way”. Our Protestant bretheren get all caught up in how they do things. They say it’s not really baptism unless you accept Jesus as your personal Lord and Savior and are immersed in water. However, there is no Biblical proof that this is the only way to baptize. Jesus said “water and spirit”. It doesn’t matter how much or how little. (If you take this far enough, you have to go to the Jordan River to REALLY get baptized…). They say we’re saved by faith alone (while John tells us that faith without works is dead), and that we get our faith from scripture alone (even though there was no New Covenant for several decades after Jesus’ church was founded). They talk about the way we Catholics worship (all the smells and bells, which incidentally are scriptural-see Revelation), Mary and the Saints, and the Eucharist.
The one thing we know is that God cannot be God if He is capable of being divided, and the Catholic Church refuses to be divided. Certainly, folks have split off, but they’re just branches of the full truth. Today’s gospel from John 15:1-8 tells us that He is the True Vine, and the Father is the Grower. That the Grower prunes dead branches because they don’t bear fruit, and live branches so that they bear more fruit. But they are all part of the True Vine. From the very time of Jesus’ public ministry, branches have fallen from His vine over hard sayings like his teaching on the Eucharist in John Chapter 6. People gave up walking with Jesus and returned to their former life (of sin) at the thought of eating His flesh and drinking His blood. But Jesus did not defer-he deepened the statement “Unless you eat of the flesh of the Son of Man and drink His blood, you have no life in you.” Judas betrayed him because he wasn’t the deliverer Judas was expecting. Then came the Gnostics, the Arians, the Pelagians, the Albigensians, the Montanists, and finally the 35,000 Protestant sects. Protestantism is divisive because there is no authority. In today’s first reading, when there was a question of who could be a Christian and who couldn’t, some said you had to be circumcised, some said you didn’t, where did the dispute fall? To Jesus’ church leaders-Peter and the other apostles. The question was brought to them, they debated it and came up with a solution, and they prayed, because Jesus had breathed the Holy Spirit into them in the Upper Room, and told them that the Holy Spirit would keep them from teaching error. They decided that circumcision was not necessary, that only baptism was necessary to become a Christian. Some didn’t like this teaching and fell away. Many times over the centuries, different things came up. Some said Jesus wasn’t really divine, some said he wasn’t really human, some said he was part man and part human, the Church council decided prayerfully that he could only be 100% man and 100% human. The doctrine of the trinity, the doctrines of Mary, and so on were all debated, and some always didn’t like the decision and fell away.
Some Catholics say it’s time for the Church to get with the times, and relent on some of her age old teachings. But if the Catholic Church were to do so, we would, by default, know that this isn’t really the Church He founded. Because he created a perfect church. An inerrant church. An infallible church. What makes this Church all these things? God does. If God can change, he’s not God. The Church of Jesus is the Church of God, perfect in her creation because of God, in spite of the humanity that runs her on earth.
Jesus didn’t say it would always be easy. Some found out when they lost their lives because of their faith. Some when they were persecuted, denied work or basic subsistence because of their faith. But the one Church Christ founded is indestructable. Nero couldn’t keep it down, neither could Hitler, Pol Pot, Stalin, Mao or the Muslim hoardes, the Goths, Vandals, Visigoths, Huns or Vikings. In fact, the Church opened her arms to most of these groups and assimilated them in to the Way, the Truth and the Life. Our arms are not closed to our Protestant bretheren, in fact we pray for them to come back home, like the Prodigal Son-welcomed with open arms and the fatted calf.