Where can I read the oldest and purest texts of Buddhism? I happen to read this text from the Buddhist Catechism (which was written by Olcott, published 1881):

Q. What striking contrasts are there between Buddhism and what may be properly called &quot;religions?&quot;
A. Among others, these: It teaches the highest goodness without a creating God; a continuity of life without adhering to the superstitious and selfish doctrine of an eternal, metaphysical soul-substance that goes out of the body; a happiness without an objective heaven; a method of salvation without a vicarious Savior; redemption by oneself as the Redeemer, and without rites, prayers, penances, priests or intercessory saints; and a summun bonum, i.e., Nirvâṇa, attainable in this life and in this world by leading a pure, unselfish life of wisdom and compassion to all beings.

But any recent texts I read about Buddhism all make it appear as if Buddhism is no different from any other dogmatic religion. I understand that Buddhism also underwent a lot of changes due to local cultures and competition from other religions.
What I want to learn is the original core message of Buddhism before it was made into a religion. Where can I find texts for understand this message.
The original teachings are found in the Pali suttas, where the Refuge in the Dhamma (Teachings) is described as follows:


  The teaching is well explained by the Buddha—visible in the here &amp;
  now, immediately effective, inviting inspection, relevant, so that
  sensible people can know it for themselves.
  
  SN 16.3


You start with the 1st three sermons of the Buddha, which are: 


Four Noble Truths and Noble Eightfold Path
Three Characteristics 
Three Defilements 


Then: 


Dependent Origination 
Emptiness


Also, where the Buddha declares what exactly he uniquely teaches: 


Handful of Leaves
"Suffering and the cessation of suffering"
Emptiness
Six elements; six bases for contact; eighteen mental examinations; and four noble truths


To understand the original teachings, the following words are best understood: 

(i) "jati" ("birth") or "jatiya", which means "self/social identity". 

(ii) "marana" ("death"), which means "death of identity". 

(iii) "kaya" (often translated as "body"), which means "group" or "collection" of aggregates.

(iv) "nirodha", which can mean "cessation" but often means the cessation of ignorance &amp; craving.

(v) "beings" ("satta'), which on a spiritual level refers to thought created "idea" or "view".

(vi) "dukkha". in relation to the four noble truths, "dukkha" means "suffering" or "stress"; in relation to the Three Characteristics of the 2nd sermon, "dukkha" means "unsatisfactoriness", "ungratifying" or "unable to bring true happiness"; in relation to feeling, "dukkha" means "painful" or "unpleasant". 

Once the above is understood, you will be able to negotiate your way through the teachings. 

You can read these book to help you: 


Two Kinds of Language
Buddha-Dhamma for Students
The Shape of Suffering up to &amp; including page 6
Nibbana For Everyone
Kamma in Buddhism