Note 3 at 2 Th. 1:3: Two things in particular were the occasion for Paul's thanksgiving for the Thessalonians. First, they had grown in the Christian faith and secondly, they abounded in love for one another. This was very good news considering that they had been converted for only a very short time, probably only a few months.

Paul had only ministered in Thessalonica for less than three weeks (Acts 17:2). It speaks very well of his ministry that in this short period of time, he had laid a foundation of faith among the believers that was still holding and increasing.

Note 4 at 2 Th. 1:3: Technically it is the exercising of our faith which causes growth. That is to say, we get better and better at using the faith we have. But our faith doesn't actually grow. It's our application of faith that is growing. When we were born again, every believer was given THE measure of faith (see note 16 at Rom. 12:3, p. 826). It's the same faith in amount and power as Jesus has for it is the faith of Christ (see note 15 at Gal. 2:20, p. 1063). But we do have to grow in our knowledge and usage of faith.