God’s existence can be known by reason alone, and His attributes (perfection, love, etc.) are engraved upon the human heart (the Natural Law). This much knowledge of God is available to all men.
A deeper knowledge of God is available through revelation. God came to Earth in the Person of Jesus Christ. He was crucified, died, buried, and rose again. He ascended into Heaven, leaving behind a corps of believers headed by Saint Peter. It is through these apostles (the Church) and the truth they have revealed to us (i.e. Scripture and Tradition) that we can know Jesus Christ, and therefore God, personally.
Those who have not heard of Jesus Christ, or who have been raised in some other faith, can still be saved by Him, although no one is sure exactly how this happens. (”My sheep hear my voice: and I know them, and they follow me” = John 10:27) We do know, however, that God is perfect, and, since He is perfect, He cannot fail to give every human being exactly what he or she deserves as a result of their choices here on Earth. “Shall not the Judge of all the earth do right?” Genesis 18:25
It sounds that if God’s existence and a deeper knowledge of Him, can be known by reason and revelation, then it certainly is possible that people of different faiths, can actually both be correct.