As time passes by many things seem to evolve or change. If something about God or salvation is true today shouldn't it be true for all time since God does not change? (Mal.3:6) It is true that if you take a story and verbally pass it down through a hundred people or so the story will more than likely not be the same at the end as it was when it started. If the foundational doctrines of the church are not the same today as in the beginning then we no longer posses the truth as Jesus and the Apostles taught. This is why we must constantly go to the scriptures for answers. The bible is our written record  of the truth revealed to us by God. We are told in the bible that we should study the scriptures and also that the Holy Spirit can teach us.

2 Tim 2:15 Study to shew thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth. (KJV)

I Jn 2:27 And as for you, the anointing which you received from Him abides in you, and you have no need for anyone to teach you; but as His anointing teaches you about all things, and is true and is not a lie, and just as it has taught you, you abide in Him. (NAS)

John 16:13 "But when He, the Spirit of truth, comes, He will guide you into all the truth; for He will not speak on His own initiative, but whatever He hears, He will speak; and He will disclose to you what is to come. (NAS)

John 14:26 "But the Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all that I said to you. (NAS)

1 Cor 2:10 For to us God revealed them through the Spirit; for the Spirit searches all things, even the depths of God. (NAS)

           This same Holy Spirit that will teach us as we study to rightly divide the word of truth has also taught the people in the beginning of the church. These people at the beginning of the church also had the advantage of being able to ask the Apostles what was meant in the letters they wrote that are now our New Testament. For example Polycarp and Ignatius were both men that studied under the Apostle John who wrote what we call the gospel according to John as well as 1st, 2nd and 3rd John and the book of Revelation. We have records of what Polycarp and Ignatius wrote. When you review what was written by people that studied under the Apostles and then men that later studied under them you can see how they interpreted the scriptures. Here is where we find conformation that the church today is indeed teaching the same basic truths that were taught in the beginning of the church. There are some minor doctrines taught in churches today that are not found in the writings of these men. For example the teaching that the church will be raptured before the Great Tribulation is not confirmed in the writings of any of these men. However the foundational doctrines which the many Christian denominations hold in common are confirmed in their writings.