"Ellen MacArthur" wrote
That's an easy one to answer. It's explained in Matthew 16 verses 13-19.

13 And Jesus came into the region of Cesarea Philippi: and he asked his disciples:
who do people say that the Son of man is?
14 But they said: Some John the Baptist, some Elias, and others Jeremias, or one
of the prophets.
15 Jesus said to them: But who do you say I am?
16 Simon Peter answered and said: You are Christ, the Son of the living God.
17 And Jesus answering, said to him: Blessed are you, Simon son of Jona, because
flesh and blood have not revealed this to you, but my Father who is in heaven.
18 And I say to you: That you are Peter; and upon this rock I will build my church,
and the gates of hell shall not prevail against it.
19 And I will give you the keys of the kingdom of heaven. And whatsoever you shall
bind upon earth, it shall be bound also in heaven: and whatsoever you shall loose
upon earth, it shall be loosed also in heaven.

Now, the Catholics use verse 18 as the start of the Catholic church. But, we
Protestants disagree with the Catholics. The Catholics think Jesus said Peter was
the rock on which the church was to be built but we believe Jesus was not talking
about Peter as the rock but rather the faith that Peter demonstrated. Upon this
demonstration of faith I will build my church. That's the beginning of Christianity.

Cheers,
Ellen

I just wanted to add something. The verses above are from the Catholic Bible.
Some of us Protestants like the King James Version of the Bible better. I'm one of them.
Here it is...

16:13 When Jesus came into the coasts of Caesarea Philippi, he asked his disciples,
saying, Whom do men say that I the Son of man am?
16:14 And they said, Some [say that thou art] John the Baptist: some, Elias; and
others, Jeremias, or one of the prophets.
16:15 He saith unto them, But whom say ye that I am?
16:16 And Simon Peter answered and said, Thou art the Christ, the Son of the
living God.
16:17 And Jesus answered and said unto him, Blessed art thou, Simon Barjona:
for flesh and blood hath not revealed [it] unto thee, but my Father which
is in heaven.
16:18 And I say also unto thee, That thou art Peter, and upon this rock I will
build my church; and the gates of hell shall not prevail against it.
16:19 And I will give unto thee the keys of the kingdom of heaven: and
whatsoever thou shalt bind on earth shall be bound in heaven: and
whatsoever thou shalt loose on earth shall be loosed in heaven.