Nope. I'm asking how can you know something is true?
We know something is true by direct observation or by the testimony of others.
Religious faith in the reasonable and Catholic sense is an extension or application to the spiritual world of an ordinary intellectual process which all exercise daily, and without the exercise of which our lives as social beings would be impossible. This process consists in assenting to the truth of propositions on the testimony of others. We may acquire knowledge in two ways - either by direct observation (you see a man knocked down by a car in the street), or through the testimony of others (you read an account of the accident in the evening paper, or learn it from a friend).
The last intellectual operation, whereby we assent to the truth of facts (which are, perhaps, beyond the reach of our own observation) because other men testify to their truth, plays an incessant part in our lives. It is in this way most of our knowledge comes to us - on the authority of others. If you reflect on the method whereby people as a rule acquire scientific, geographical, historical, philosophical knowledge, or if you think of the part which books and newspapers play in our lives, you will, I think, admit the truth of what I say. We each of us investigate a very small portion of the earth's surface on which we live - namely, the part traversed by the tiny track of our perambulations through life. All the other knowledge we have of the world - or of the universe - rests on the testimony of others. That's what all education amounts to: the testimony of others.
Now, who will say that such faith, such willingness to accept testimony, is unscientific, or unworthy of a rational being? Who will suggest that it is not based on sound intellectual principles? It may not be easy for you to trace the process whereby you have come to believe without any doubt in the existence of Jupiter's satellites, or of icebergs in the Antarctic, or of Hitler or Mussolini. The evidence has come through many almost imperceptible channels, but is such that it excludes all doubt from your mind. If you analyze the process, it comes to this: You convince yourself by direct examination or reasoning of the reliability of the witness; then you accept his testimony as true.
Two things must be clear to you about the witness -
(1) That he had ample opportunity to learn the facts;
(2) that he is telling the truth.
In other words, that he is not deceived himself, nor wants to deceive you. In a court of law, the judge and jury must test these two points:
Is the witness truthful?
Has he knowledge of the facts?
Once they are convinced of these two things, then they accept his evidence, and believe his statements to be true.
To a Catholic believer Faith is just this process. It is not conjecture, nor is it credulity. It means assenting to the truth of certain facts on the evidence of a reliable witness, the witness in this case being God Himself. That the facts (e.g., the Trinity, Incarnation, the Real Presence ) are beyond our ken and cannot be directly tested by us is no more a difficulty to our accepting them (when the evidence is sufficient) than my inability to investigate directly the murder of Julius Caesar or the execution of Mary Queen of Scots militates against my belief that these two eminent persons met with violent deaths.
To put it another way, assenting to the truth of Julius Caesar's murder, or Mary Queen of Scots execution is not difficult for me to accept as true (even though I have not personally observed these events). This is because assenting to the truth of certain facts comes to us by the testimony of others.
Such faith, such willingness to accept testimony, is scientific, and worthy of a rational being. It is based on sound intellectual principles, contrary to the constant drumbeat that all faith is blind acceptance, credulity or sentimentalism.
(gleaned from a paper by Fr. Albert Power S.J. 1960, not available on line)