Answer:
Protestant Churches are Christian Churches. However, the Roman Catholic Church, established in the 300s AD, does not technically recognise the various Protestant denominations as Christian, as it believes it is the only true Christian faith.
Protestantism came about because of what Martin Luther and other Roman Catholic clergy considered as corruption within the Church. Luther himself did not want a schism (a split) but, rather, he wanted to reform the Church as it was. His suggested reforms were rejected by a majority within the Church and so we now have the two main streams or splits, both of which are directly descended from the original Church established by Jesus through Peter "the rock".
The Roman Catholic Church itself was one split of several, from the original foundation of the Church after the crucifixion of Jesus.
Both the Roman Catholic Church and the various Protestant Churches are Christian Churches, as they believe in the existence of Christ and that Christ died on the cross for our sins. The differences between them is the interpretation of scriptures - technicalities. Some denominations take the teachings and scriptures more literally than others, for example.
Protestant opinion:
in my opinion Catholicism is little more than an idol worshiping cult that places mere human beings above Jesus and wishes to control humanity through a political system, if you don't believe me study the 'thirty years war', the Borgias and the way their church is structured, so to my mind yes the only Christians are protestant but you will get a different answer from Catholics or even other protestants however not all Catholics are bad in fact some of them are wonderful people and some of the denominations aren't as psychotic as the standard. maybe I'm just biased and wrong but the blood of my ancestors is on their hands and I don't approve of tyranny.
Roman Catholic answer: Christianity was started by Jesus Christ and the Holy Spirit at Pentecost when He (the Holy Spirit) descended on the Apostles and "confirmed them in all truth". They were the first Bishops, with Peter as their head as the first Pope. Down through the centuries, the Bishops, in union with the Holy Father (the Pope) have held the faith of the apostles in its completeness. The Protestant churches were breakaways, fifteen centuries later, who came along denying certain aspects of the faith which was always and everywhere believed. Protestants are considered Christian in so far as they believe in Jesus Christ and Baptism.