In our pluralistic society a common attitude is that it doesn't matter what church you belong to as long as you love Jesus. I will agree that one's eternal salvation is a mystery and I treat everyone who follows after Jesus as a Christian brother or sister whether they are Catholic or not. Many evangelical and protestant authors have written books that I've benefited from. The prayers of my non-Catholic family and friends are always welcome.
Yet, the Body of Christ is not some fragmented nebulous conglomeration of conflicting interpretations of Scripture (1 Corinthians 12:13). The Church is the pillar and ground of truth (1 Timothy 3:15) built on the foundation of the Apostles and Prophets (Ephesians 2:20). The authority given to the Apostles was passed on to faithful men (2 Timothy 2:2) who hold the traditions they were taught orally and in writing (2 Thessalonians 2:15).
Who fulfills all of these criteria? Only the Catholic Church (and I include the Eastern Orthodox under this umbrella) can claim Apostolic succession and uninterrupted faithfulness to Apostolic doctrine. The Church is living and breathing, it adjusts and grows over time, but it's roots and fruits remain the same.
Here's why it matters: Jesus promised that He would preserve the Church and gave it authority (Matthew 16:18-19). As part of the Catholic Church I come under this protection. I have the assurance that the Fathers and the Magisterium are guided by the Holy Spirit to keep me from error (John 16:13). I can receive the divine presence of Christ through Communion (John 6:53-54) and the removal of sin and renewal of my relationship with God through Reconciliation (1 John 1:9). I am part of a 2000 year old, world-wide spiritual family!
I'm sorry if my triumphalism makes you uncomfortable, but these are the facts. As Catholics we have so many resources to assist us as we work out our salvation with fear and trembling (Philippians 2:12). Other "ecclesial communities" might have zeal (Romans 10:2), heartfelt worship (John 4:24), and television ministries (Luke 9:49-50)--but they can't claim the authority and protection that exists in the Catholic Church.
It does matter!