Translation
as the testimony concerning CHRIST has been confirmed in you, (See comment below.)
Paraphrase
as you have done many things which confirm your testimony about what THE ANOINTED MESSIAH has done in you, (See comment below.)
SOME OF THEM HAD A GOOD TESTIMONY
Some of them were living in a way that proved they had been transformed and the Holy Spirit was residing in them and guiding their lives. This cannot refer to those who were doing the opposite of what they were taught about following Jesus.