If philosophy hasn't given you the certainty that such a thing is possible, then you cannot make a decision in life. It is this binding quality of truth which Veritatis Splendor writes about. Individualism, in the extreme, leads to a denial of authentic human nature. If we claim that everyone can invent his own truth, this leads to moral chaos. Although each individual has freedom, there exists a prior obligation to seek the truth and to adhere to it once it is known.
Truth ought also to be shared.
You're calling your friend into the contemplation of a great beauty. Just as with the perceived beauty to continue a difficult hike to a beautiful vista, so with the intellectual pursuit of truth. The positioning by Saint Thomas of the intuitive cognition of God as the summit of moral action is done to help us to draw everything together toward God. He proposes such an opening of our intellect, so that we can obtain happiness. When we approach the text of Saint Thomas with the modern understanding of contemplatio, we misunderstand him.