Christians are not a monolithic group of people. While they share many beliefs and traditions in common, there are still several things in which they differed one from another. Some are a mater of preferences, for example, their choice of contemporary verses traditional styles of worship. Some are doctrinal issues, such as how and when they baptize people. However, with all these differences they are still "believers" and "Christians" and we would still call them brothers and sisters in Christ.
However, there are some things that are non-negotiable. Some beliefs that are so central to the Gospel that to deny them is to place yourselves out side of orthodoxy; to place yourself outside of Christianity. Polycarp identifies some of the central tenets of our belief.
" 'For whosoever does not confess that Jesus Christ has come in the flesh, is antichrist;' and whosoever does not confess the testimony of the cross, is of the devil; and whosoever perverts the oracles of the Lord to his own lusts, and says that there is neither a resurrection nor a judgment, he is the first-born of Satan." (Polycarp 7)It is interesting that what Polycarp considers essential to our beliefs as Christians all relate to the person of Jesus. We can differ in many ways and even get many things wrong, but its important that our understanding and belief in the person of Jesus is sound and true.
Specifically, Polycarp mentions four truths about Jesus. First, He was born a man. He was not a phantom. He was not some spiritual aberration. He was fully man and fully God. He was God in human flesh. Secondly, He suffered and died on the cross for our sins. He was not insulated from the pain and suffering of His crucifixion nor did His "god-ness" escape just before His flesh died lest it too felt death. He suffered and died in our place; the righteous for the guilty so that the guilty may go free. Third, He was raised from the dead, not as a ghost, but bodily; He could be touched, held, and could eat. His resurrection gave proof of who he claimed to be, the Son of God, and gave us hope that we too will one day share in His resurrection. His resurrection also demonstrated the power that was within Him and that now also works in us to will and do His purpose. Forth, and finally, He will one day return to judge the living and the dead. We will all face judgement before His throne; some to everlasting life in His presence and some to everlasting judgement away from His presence.
Beyond this, most of the teachings of the Apostles are centered around how we should live towards one another and towards God. These are the central truths of the Gospel and Polycarp exhorts us to continue in them and in the things we have been taught.
"Wherefore, forsaking the vanity of many, and their false doctrines, let us return to the word which has been handed down to us from the beginning; 'watching unto prayer,' and persevering in fasting; beseeching in our supplications the all-seeing God 'not to lead us into temptation,' as the Lord has said: 'The spirit truly is willing, but the flesh is weak.' " (Polycarp 7)Having believed, let us continue in God and in our pursuit of Him.