Now Jesus commissioned His apostles, the leaders of His Church, to teach the Gospel to all nations. Till this time ONLY the apostles had received all the teachings, the entire Gospel.
But, no where in the bible do we have a summary of this Gospel that they taught. In a few paragraphs we have a little of the creed, that is it. In the written Gospels we have some of the teachings of Jesus, before His death, but that is it. Jesus taught and trained His apostles for 3 years. We have none of those teachings He taught them in secret. We have none of those teachings in which He taught them how to conduct a worship service, or the sacraments, or how to pray, or the Trinity.etc. We have no summary of what the Holy Spirit taught the apostles.
We have no summary, anywhere, of the Gospel the apostles taught and preached. This is the real reason why Protestants can’t know the Gospel of Jesus Christ. They think they can get it from Scripture, but no where does Scripture claim to have even a summary of the Gospel the apostles taught and preached. Scripture is basically salvation history, NOT a summary of the Gospel the apostles taught and preached.
So how do we know what the apostles taught and preached? We can learn that from those who learned from them. In other words, the early Church Fathers leaned the Gospel from the apostles. If we study their writings, we learn that all Christian writers believed that bapstism was salvific, that the mass was a sacrifice, that the bread and wine become the body, blood , soul and divinity of Jesus, that we should all be confirmed after baptism, that we can lose our salvation by mortal sin, that we should pray to the saints, etc.
In other words, the Gospel the apostles taught and preached is contained in the Church teachings today. The catechisms of the Church contain the exact same Gospel the apostles taught and preached. And since the apostles could not teach the Gospel in the same form they learned it, that is teachings of Jesus before His death, then after His death, then those of the Holy Spirit, the apostles organized these teachings in order for those who heard them, to learn and remember them.
So they early Church organized the Gospel into the Creed, (the basic beliefs), the sacraments (how we receive the grace Jesus merited by His death and resurrection) , the commandments, (the moral life we are to live, in order to reach heaven) and prayer, (the Our Father, and prayer to the saints)
Thus, when the apostles taught the Gospel, then simply taught the teachings of the Creed, Sacrements, Commandments and Prayer. In other words, Catechisms are basically the Gospel the apostles taught and preached. Of course, by their example and their way of worship, by the institutions they founded, etc., they also handed down the Gospel.
One may ask, if everything is in the Catechisms, and only some is in the bible and even this is rarely explict and clear, then why have the scriptures?
The scriptures present salvation history. They present a historical context of the teachings of God. They also present an accurate glimpse of how Jesus acted Himself and how He taught and some of His direct teachings. And since they portray salvation history, then we can use scripture to nourish the teachings of the Gospel, to illuminate the teachings of the Gospel, and to be a witness to the teachings of the Gospel, and even a few times to be a witness to teachings as a proof of these teachings. The words here in bold are the exact words the Church uses to refer to scripture in its relation to Tradition.
But again, to learn the Gospel the apostles taught, we must
be taught in an organic and systematic way, just as the apostles always taught the Gospel in an organic and systematic way. And today that is the Creed, Sacrments, Commandments and prayer. That is why the Church says the Catechism is a sure norm for teaching the faith. When we teach the teachings of the Catechism, we are handing on the Gospel of Jesus Christ. That is the only way it can be learned.