Well, we can know we have the true Faith by looking at the evidence. Look at the Bible and the Early Church Fathers. See how Catholic they are? That is some sure evidence that the Catholic Church is the true Faith. But most importantly, we must have faith that we are in the true Faith. Christ founded the Catholic Church and made Saint Peter it’s first leader, the first pope. Christ also promised us that the gates of Hell would never prevail against the Church. We must trust His words.
(Matthew 16:13 RSV) Now when Jesus came into the district of Caesare’a Philip’pi, he asked his disciples, “Who do men say that the Son of man is?”
(Matthew 16:14 RSV) And they said, “Some say John the Baptist, others say Eli’jah,
and others Jeremiah or one of the prophets.”
(Matthew 16:15 RSV) He said to them, “But who do you say that I am?”
(Matthew 16:16 RSV) Simon Peter replied, “You are the Christ, the Son of the living God.”
(Matthew 16:17 RSV) And Jesus answered him, “Blessed are you, Simon Bar-Jona! For flesh and blood has not revealed this to you, but my Father who is in heaven.
(Matthew 16:18 RSV) And I tell you, you are Peter, and on this rock I will build my church, and the gates of hell shall not prevail against it.
(Matthew 16:19 RSV) I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven.”
So yeah, we know that the Catholic Church is the one true Church and the one true Faith by these things. Plus, we know that Christianity is the one true Faith because Jesus said:
(John 14:1 RSV) “Let not your hearts be troubled; believe in God, believe also in me.
(John 14:2 RSV) In my Father’s house are many rooms; if it were not so, would I have told you that I go to prepare a place for you?
(John 14:3 RSV) And when I go and prepare a place for you, I will come again and will take you to myself, that where I am you may be also.
(John 14:4 RSV) And you know the way where I am going.”
(John 14:5 RSV) Thomas said to him, “Lord, we do not know where you are going; how can we know the way?”
(John 14:6 RSV) Jesus said to him, “I am the way, and the truth, and the life; no one comes to the Father, but by me.
(John 14:7 RSV) If you had known me, you would have known my Father also; henceforth you know him and have seen him.”
(John 14:8 RSV) Philip said to him, “Lord, show us the Father, and we shall be satisfied.”
(John 14:9 RSV) Jesus said to him, "Have I been with you so long, and yet you do not know me, Philip? He who has seen me has seen the Father; how can you say, ‘Show us the Father’?
(John 14:10 RSV) Do you not believe that I am in the Father and the Father in me? The words that I say to you I do not speak on my own authority; but the Father who dwells in me does his works.
(John 14:11 RSV) Believe me that I am in the Father and the Father in me; or else believe me for the sake of the works themselves.