Isaiah saw the glory of the Lord and was distressed by the lack worship and trust in God. Nicodemus saw the signs Jesus did and approached Him. God calls all of us to Himself... Is our heart open to say "Here I am send me."
The church was in existence before the Day of Pentecost but on this day God empowered it to go forth into the world. Jesus had commissioned His disciples on the day of the resurrection and now the whole body was being empowered to go forth into the world.
The disciples had come to fully recognize and were convinced that Jesus was the Messiah and was God's Son. Jesus knew they would face persecution by the world and would face temptation to walk away from believing. Jesus prays for their protection and that God would keep them in truth of who He is and what He did for the world.