The phrase can be understood to mean that Christ Jesus is the living environment within which their faith is exercised.
1 Though there may be some truth to this, it will be proper to take this statement to refer to Christ as the object of faith. Paul uses this phrase and ones similar to it to indicate the general idea of a faith that has Jesus Christ as its ultimate reference point.2
4 since we heard of your faith in Christ Jesus and of the love that you have for all the saints,