Are Catholics Christians?
The short answer
Yes. Catholics are Christians — in fact, the Catholic Church is the oldest and largest Christian community. Catholics believe in the Trinity, confess Jesus Christ as Lord and Saviour, are baptized in his name, and hold the Bible as the inspired Word of God.
What makes someone a Christian
The disciples "were first called Christians" in Antioch (Acts 11:26) — followers of Jesus Christ. Historically, Christians are those baptized in the name of the Father, Son and Holy Spirit who confess Jesus as Lord. Catholics share every element of that definition.
What Catholics believe about Christ
The Catholic Church professes the Nicene Creed every Sunday: Jesus Christ is "true God from true God," who "for us men and for our salvation came down from heaven," was crucified, rose on the third day and will come again. Salvation comes through him alone: "There is no salvation through anyone else" (Acts 4:12; CCC 432).
The oldest Christian Church
The phrase "the Catholic Church" appears in the writings of Saint Ignatius of Antioch around A.D. 107. For the first thousand years of Christian history there was essentially one Church in East and West; the major divisions came in 1054 (East–West) and the 1500s (the Reformation).
Why the question comes up
Some Christians think Catholic practices — honouring Mary, praying for the dead, the sacraments — add to or replace faith in Christ. Catholics see each of these as flowing from Christ. Real differences exist between Catholics and other Christians, but the Catholic Church calls baptized Protestants and Orthodox "brothers [and sisters] in the Lord" (CCC 818).
A shared foundation
"one Lord, one faith, one baptism; one God and Father of all" (Ephesians 4:5-6). What unites Christians is greater than what divides them — which is why the Catholic Church prays and works for Christian unity.