God implants Himself in the interior of that soul in such a way that, when it returns to itself, it cannot possibly doubt that God has been in it and it has been in God; so firmly does this truth remain within it that, although for years God may never grant it that favor again, it can neither forget it nor doubt that it has received it (and this quite apart from the effects that remain within it, and of which I will speak later). This certainty of the soul is very material.
But now you will say to me: How did the soul see it and understand it if it can neither see nor understand? I am not saying that it saw it at the time, but that it sees it clearly afterward, and not because it is a vision, but because of a certainty which remains in the soul, which can only be put there by God. (Teresa of Avila: Fifth Mansions: Chapter One)
We can authenticate our mystical experiences. We do this by assessing the quality of the outcomes and by consulting wise spiritual guides. But, the most important authentication is the certitude within. While it’s difficult to assess the mystical event itself, there is a “certainty which remains in the soul” after the event occurred. This certainly “can only be put there by God.” Even if the soul never has another mystical experience, we’re certain this one experience came from Him.
Teresa tells the story of a person (herself) that received a spiritual favor. She learned a great truth from God and was convinced firmly of this truth. A “half-learned” spiritual guide contradicted this truth. Teresa says, “she had the truth so firmly implanted within her that she did not believe him, and asked others, who told her the truth”. This, of course, does not mean we search until we find someone to agree with us. But we need to know when to consult richer sources. We err when we defer to those who know less than we do.
How about you? Is there a deep spiritual experience in your past you’ve never fully explored. I encourage you to study it. Assess the quality of the outcomes. Consult wise guides. But most important, be attentive to the certainty placed deep within your soul.