"When he comes, the Spirit of truth, he will guide you to all truth"
What is the key to knowledge if not the grace of the Holy Spirit conferred by faith? Its illumination truly gives knowledge, divine knowledge, and opens our closed and veiled minds, as we frequently experience with regard to many parables and figures, to say nothing of clearer demonstrations. Pay careful attention, therefore, to the spiritual meaning of the word. If the key does not open the door - for Scripture says, to him the doorkeeper opens - it remains unopened; and if the door is not opened, no one enters the Father's house. As Christ says: "No one comes to the Father except through me" (Jn 14:6).
Now the fact that it is the Holy Spirit who first opens our minds and teaches us about the Father and the Son is again stated by Christ himself: "When the Spirit of truth comes, who proceeds from the Father, he will bear witness to me and he will lead you to the whole truth" (Jn 15:26; 16:13). Do you not see how through the Spirit, or rather in the Spirit, the Father and the Son come to be known inseparably?...
The Holy Spirit is called the key because it is through him and in him that we first receive spiritual illumination, and being purified, are enlightened with the light of knowledge, and baptized from above, and born again, and called children of God. As Paul says: "The Spirit himself intercedes for us with inexpressible groans" (Rm 8:26); and again, "God has given his Spirit to our hearts and he cries out, 'Abba, Father' " (Gal 4:6). It is the Spirit, therefore, who shows us the door which is light, and the door teaches us that he who inhabits the dwelling is himself also light inaccessible.
St. Margaret Mary Alacoque