Jn 16:12-15
At that time, Jesus said to his disciples: I have yet many things to say to you, but you cannot bear them now. When the Spirit of truth comes, he will guide you into all the truth; for he will not speak on his own authority, but whatever he hears he will speak, and he will declare to you the things that are to come. He will glorify me, for he will take what is mine and declare it to you. All that the Father has is mine; therefore I said that he will take what is mine and declare it to you.
While still on earth with His disciples, the Lord did not entrust everything to them, knowing that the Holy Spirit had to be sent to them first, both to allow them to understand things and to strengthen them to put them into practice.
This is how the Lord guides His Church through time, enabling it to discern His revelation through the Holy Spirit ever more clearly.