In my sermon on the Apocrypha, Saints, and the Blessed Virgin (Catholics Have a Bigger Bible), I began by clarifying that Catholics are Christians. Though it seems odd to even have to say it, it did need to be said.
But you might ask how it is that someone judges whether or not another denomination or church are Christian, and that is an important question. It is important to take a moment to figure out what is the bottom-line list of non-negotiables that decides whether or not an organization is a Christian church or some sect or cult or other religion altogether.
I am far from the person to make that call, but luckily, I don't have to. A long time ago (not in a galaxy far, far away) Christians had to face this question and come to a consensus. Their consensus was the Nicene Creed, and has been the key standard for orthodoxy for centuries upon centuries. Such an important document is worthy of being quoted in full:
We believe in one God,
the Father, the Almighty,
maker of heaven and earth,
of all that is, seen and unseen.
We believe in one Lord, Jesus Christ,
the only Son of God,
eternally begotten of the Father,
God from God, Light from Light,
true God from true God,
begotten, not made,
of one Being with the Father.
Through him all things were made.
For us and for our salvation
he came down from heaven:
by the power of the Holy Spirit
he became incarnate from the Virgin Mary,
and was made man.
For our sake he was crucified under Pontius Pilate;
he suffered death and was buried.
On the third day he rose again
in accordance with the Scriptures;
he ascended into heaven
and is seated at the right hand of the Father.
He will come again in glory to judge the living and the dead,
and his kingdom will have no end.
We believe in the Holy Spirit, the Lord, the giver of life,
who proceeds from the Father and the Son.
With the Father and the Son he is worshiped and glorified.
He has spoken through the Prophets.
We believe in one holy catholic and apostolic Church.
We acknowledge one baptism for the forgiveness of sins.
We look for the resurrection of the dead,
and the life of the world to come. Amen.
Wow. Simple, beautiful, profound. This is so important that most Christian worship services recite it (or some version of it) every time they gather for worship. If a group believes something in conflict with this, it is generally regarded as not Christian. But what about the Saints, the Virgin Mary, and the extra books the Catholics have in their Bible? You can watch that video here.