simple — believe in the scriptural tenets listed in the Nicene Creed. Do you believe that or not? If not, then you’re not Christian.
Or better yet, the Apostles Creed that is also almost universally accepted by Protestant denominations. It states the belief in two separate articles that, when considered together, completely undermine the single claim Protestants make against the Church. These two articles are the belief in one Catholic and Apostolic Church, AND the communion of saints.
Protestants would have you believe that they are one in the same, but that isn't the way the Early Church Fathers or the Apostles believed. Jesus left us the Church for the communion of saints. Apply this litmus test and you will see how few Christians we are actually dealing with.