Around the same time as the rise of Christianity people were growing dissatisfied with traditional religions. Various cults called the mystery religions rose up. These cults included:
-The Eleusinian mysteries (Demeter & Persephone)
-The Isaic cult (Isis)
-The Dionysian cult (Dionysus)
-The Orphic mysteries (Orpheus)
-The Cybele cult (Cybele & Attis)
Most of these cults featured a dying-and-rising god. Most promised a better afterlife for members. All required initiation in order to belong. This differed from the pagan religions of the time. They all taught secret knowledge revealed only to the worthy.
Many educated people have recognized that Christianity fits all the prerequisites for it being just another mystery religion. The initiation is one's baptism. There is a promise of a better afterlife. And yes, there are secret teachings known only to a chosen elect. These teachings are being withheld from the public.
So Jesus is admitting that he is giving secret teachings to his Apostles while the masses, the public, gets only parables. A similarly worded passage is found in Mark 4:11-12.
In 1 Corinthians Paul tells the church at Corinth that he has givin them "milk" because they are "infants". This milk is the standard teachings of Christ's crucifixion and resurrection. Paul says he is withholding "meat" because they aren't old enough to eat it, meaning they aren't yet worthy to receive the mysteries, the secret teachings. Paul repeats the same message in Hebrews 5:11-14.
1 Corinthians 3:2
I have fed you with milk, and not with meat: for hitherto ye were not able to bear it, neither yet now are ye able.
So we know that Christianity contains secret teachings that the masses are not being taught. We know that Paul knew the secret teachings. We know that a few church elders of the past, such as Clement of Alexandria, knew the secret teachings. Clement actually says the teachings are only for a select few who are being "perfected".
You will not hear this stuff being preached in today's churches. The majority of Christians are not worthy of this teaching, according to Jesus. Most people don't want to hear that they are mere children and can't handle the truth.
So what is the truth? What are the secret teachings?
To uncover the secret teachings we have to look to Gnostic Christianity.
The secret teachings of Christ most likely deal with reincarnation and the soul's journey in the afterlife, as well as it's goal of merging with the god head.
The Gnostic Christians believed that humans have a small piece of God, a spark of light, within them. The spark needs to rejoin with God. However, humans are fallen into matter and must be redeemed through various incarnations. Those that make it will unite with God and gain immortality. Those that don't will wither away, through various incarnations, until the spark goes out and they disappear into oblivion. Think of it like sperm. During sex, millions of sperm are released into the woman. Only one can fertilize the egg. The rest die. Likewise, only a few will ever unite with God. The rest will wither away through various incarnations until their spark goes out, their soul is extinguished at which point they no longer exist. So there is no real hell, no eternal torment. There is only immortality for those who make it and oblivion for those who do not.
So the question is, if Christianity meets all the definitions of being a mystery religion, then why don't modern New Testament scholars classify it as such? The reason being is that most of them are believers, Christians. They know that to admit that Christianity is a mystery religion would open up a can of worms they don't want to deal with. The biggest problem is that the mystery religions aren't based around a person who actually existed. Demeter didn't exist. Orpheus didn't exist. Cybele didn't exist. Instead, it's all about allegory and symbolism. This may be shocking to some, but the earliest Christians did not believe that Jesus ever existed in the flesh. They believed he was a heavenly messiah who would soon come to earth but had not come to earth yet. These earliest Christians were destroyed when the literalists rose to power and deemed them to be heretics and killed them.
I want to end with Dionysus. Research the myth of Dionysus. In his myths he traveled the world teaching people how to cultivate grapes and make wine. He was even said to have traveled as far away as India. His myths treat him as if he was a real man who walked the earth, turning water into wine and performing other miracles. However, we know he never existed. Ponder that.
What do you think the secret teachings of Christ were?