Although Jesus forewarned the disciples of impending pressure from the world, Jesus reiterated one of His promises and gave the disciples another call.
Jesus proclaimed that after the Father sends the disciples the Holy Spirit, the Spirit "will testify of [Jesus]" (15:26). According to Merriam-Webster, "testify" means "to serve as evidence or proof" or "to express a personal conviction", so in essence the Holy Spirit reveals who Jesus is for others. Even today, the Holy Spirit reveals Jesus -- especially when the Holy Spirit works within lives to transform them inside-out!
Speaking of testify, Jesus also called the disciples to bear witness "because [they] have been with [Jesus] from the beginning" (15:27). By then the disciples have seen a lot of Jesus's character, and they are very aware of who Jesus is. Thus, the disciples are important witnesses because they can show Jesus via first-hand experience. Jesus also reiterated that call after the resurrection but before his ascension (in Acts 1).