What Is Authentic Buddhist Teaching?

Any teaching that does not hold the Four Noble Truths, the Noble Eightfold Path, and the Three Marks of Existence is not authentically Buddhist.

The Four Noble Truths:

1...The Truth of Dukkha
2...The Cause of Dukkha
3...The Cessation of Dukkha
4...The Path to the Cessation of Dukkha

The Noble Eightfold Path

1...Skillful Understanding
2...Skillful Thought
3...Skillful Speech
4...Skillful Action
5...Skillful Livelihood
6...Skillful Effort
7...Skillful Mindfulness
8...Skillful Concentration

The Three Marks of Existence

1...Impermanence
2...No-Self
3...Dukkha

Note:
The Pali word 'dukkha' may be translated as anxiety, stress, unsatisfactoriness, discomfort, or (weakly) suffering.