Take hold of the eternal life. That is, do not become complacent. While faith in Christ begins new life (4:8), the goal of the Christian life is always future (v. 19; 1:16 note; Phil. 3:12; 2 Tim. 4:8).
to which you were called. Eternal life is not something we naturally choose, but something to which God supernaturally calls us (Rom. 8:30).
good confession in the presence of many witnesses. This probably refers to Timothy’s baptism. The “good confession” that one has come to faith in Christ leads naturally into the “good fight” of seeking to live in faithfulness to Him.