For this reason.
To test you.
Whether you are obedient
εἰς τοῦτο γὰρ καὶ ἔγραψα, ἵνα γνῶ τὴν δοκιμὴν ὑμῶν, εἰ εἰς πάντα ὑπήκοοί ἐστε.
Paul said that he wrote to them for this reason (εἰς τοῦτο γὰρ καὶ ἔγραψα). He wanted to test them (ἵνα γνῶ τὴν δοκιμὴν ὑμῶν). He wanted to know whether they were obedient in everything (εἰ εἰς πάντα ὑπήκοοί ἐστε). Only the Pauline letters used this word δοκιμὴν, that means a trial, proving, approval, or approved character. Paul explained that he wrote this letter to test them to see if they were obedient to him in everything that he asked. He wanted to see how loyal they were to him. This was their test or trial. How were they treating each other with Christian love or agape? How do you treat people with Christian love?