In Colossians 1:3-5, Paul writes of his thanks to God (ESV translation):
We always thank God, the Father of our Lord Jesus Christ, when we pray for you, since we heard of your faith in Christ Jesus and of the love that you have for all the saints, because of the hope laid up for you in heaven. Of this you have heard before in the word of the truth, the gospel
He seems to write two time clauses: when we pray and (possibly) since.
The ESV implies that the reason that Paul gives thanks is because of the hope laid up in heaven and not the faith and love of the Corinthians, but the Greek - dia - is normally interpreted as through and not because of.
Which of the two is Paul's cause for giving thanks to God: the faith and love of the Colossians or the hope laid in heaven? I feel this is important to understand here, as it affects the understanding of Paul's small tangent in v.4-8.