2 Timothy 2.2 Things .. heard = 1) either the total gospel message itself, or 2) the charge to Timothy to preach the gospel. Witnesses = many had heard Paul's preaching, and/or many witnessed Timothy's charge. Many however understand this to mean what Paul has preached from many witnesses, like Moses and the Prophets, etc. Διὰ through the medium of, OR in the presence of. Among many witnesses (dia pollōn marturōn). .. dia in this sense ... suggests that it is a legal phrase “ supported by many witnesses.” Not mere spectators, but testifiers. See Paul’ s use of dia 1 Thessalonians 4:2; 2 Corinthians 2:4; Romans 2:27; Romans 14:20. Paul in 1 Corinthians 15:1-8 gives many witnesses of the resurrection of Christ. (Robertson's Word Pictures) Be able = compentent men. Thayer, "suficient in ability." Teach others = the plan of propagating the truth, hence, "preacher-training" schools.