All truth

John 16:12-15

There is no reason why any doctrines should be kept secret from the novice and revealed to the grown up believers. Spiritual men should not altogether to withhold spiritual doctrines from the worldly, seeing the Catholic faith to be preached to all; nor should they water it down in order to accommodate them to the understanding of persons who cannot receive them, making their preaching contemptible, instead of making the truth intelligible.

So these words of our Lord do not refer to certain secret doctrines which if the teacher revealed, the disciple would not be able to bear, but to those very things in religious doctrine which are within the reach of all of us.

While in the body we cannot know all the truth, as the St. Paul says, "We know in part" (1 Cor 13:9); but the Holy Spirit sanctifying us makes us fit for enjoying that fullness of which the same Apostle says, "Then face to face" (1 Cor 13:12). Our Lord's promise: "But when he comes, the Spirit of truth,he will guide you to all truth", does not refer to this life only, but to the life to come, for which this complete fullness is reserved. The Holy Spirit both teaches believers now all the spiritual things which they are capable of receiving, and also kindles in their hearts a desire to know more.

But it is not true, as some heretics have thought, that because the Son receives from the Father, and the Holy Ghost from the Son, that the Holy Ghost is inferior to the Son. Jesus Himself solves this difficulty, and explains His own words: All things that the Father has are Mine; therefore said I, that He shall take of Mine, and shall show it to you.

References:
St. Augustine
Tractates on the Gospel of St. John
Tractate #96
Tractate #97
Tractate #98