A.T. Robertson Commentary Philippians 2:22

"But ye know the proof of him, that, as a child [serveth] a father, [so] he served with me in furtherance of the gospel." — Philippians 2:22 (ASV)

The proof (την δοκιμην). "The test" as of metals (2 Corinthians 2:9; 2 Corinthians 9:13). Three times they had seen Timothy (Acts 16:13; Acts 19:22; 20:3f.).

With me (συν εμο). Paul's delicacy of feeling made him use συν rather than εμο alone. Timothy did not serve Paul.

In furtherance of (εις). See Php 1:5 for this use of εις.