The Catechism in Questions & Answers

201. What does it mean to say that “the Holy Spirit is the Spirit of truth”?

The Holy Spirit makes clear what is pleasing to God and that which opposes God’s will. As the Spirit of truth, it identifies truth and falsehood. The Holy Spirit sees to it that the message of the sacrificial death, resurrection, and return of Christ is preserved and spread throughout the ages.

“But when the Helper comes, whom I shall send to you from the Father, the Spirit of truth who proceeds from the Father, He will testify of Me.”
John 15: 26