I Corinthians 1:6

Even as the testimony of Christ was confirmed in you (KJV)

Then: Nothing can be more comforting than to have someone else see Christ in you. It removes self-doubt. It removes concern that maybe something is the matter with your rock solid belief that Jesus is the Christ, risen from the dead, the Son of God. And to have Paul, an apostle, thanking God for your belief has to be a major epitome of God's grace to you. The verse is also another statement of the past event which N. T. Wright (p.4) calls a "messianic message." Christ is evident in the Corinthians because his name is readily spoken in love for him, in prayer through him, and friendship with him. Matthew Henry infers that "testimony" means the presence of signs of wonders which verify the doctrine of Christ. In such instances believers maintained deep fellowship with Christ and with each other.

Now: In times when the bond with Christ is solid, prayers answered, peace and grace abide, a believer doesn't need outside confirmation of his/her Christ-like image. But on days when faith wobbles and doubt intervenes, a friendly statement about obvious choices to serve the Lord by upholding his standards go a long way to removing self-doubt. A continuous, thankful review of all God's past work in our lives, lives of friends, and life of our church can return faith to its solid state. It orders our memories and our priorities. It becomes a major part of fellowship with Christ and other believers.

Extra Source: Commentary on the Whole Bible (Acts to Revelation) by Matthew Henry, see: http://www.ccel.org/ccel/henry/mhc6.iCor.ii.html