Ephesians 1:13

In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also having believed, you were sealed with the Holy Spirit of promise.  

The word of truth is the word of God that brings us to salvation.  Our trust in Jesus and the promises written in the Bible lead us to confidence in our faith.  Our faith that we have an eternal future with Jesus in heaven and the Holy Spirit to guide us on the way.