An overseer, then, must be above reproach, the husband of one wife, temperate, prudent, respectable, hospitable, able to teach, …
1 Timothy 3:2 (emphasis mine)
Herein is the major distinction between elders and deacons. Deacons may be able to teach, but elders must be able to teach. A failure in this regard is a disqualification for eldership.
The question that must be wrestled with is what exactly does it mean to be able to teach? We can draw a number of conclusions by considering the vital responsibilities of elders that relate to teaching.
An elder must be sound in his theology, he must be able to exhort in sound doctrine, and refute those who contradict the truth.
… holding fast the faithful word which is in accordance with the teaching, so that he will be able both to exhort in sound doctrine and to refute those who contradict. For there are many rebellious men, empty talkers and deceivers, especially those of the circumcision, who must be silenced because they are upsetting whole families, teaching things they should not teach for the sake of sordid gain.
An elder must be faithful in sharing the gospel and discipling others. This includes being able to disciple other men to be elders.
And Jesus came up and spoke to them, saying, “All authority has been given to Me in heaven and on earth. Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit, teaching them to observe all that I commanded you; and lo, I am with you always, even to the end of the age.”
The things which you have heard from me in the presence of many witnesses, entrust these to faithful men who will be able to teach others also.
2 Timothy 2:2
An elder must be capable of equipping the saints for the work of service and assisting them in maturing in the faith.
And He gave some as apostles, and some as prophets, and some as evangelists, and some as pastors and teachers, for the equipping of the saints for the work of service, to the building up of the body of Christ; until we all attain to the unity of the faith, and of the knowledge of the Son of God, to a mature man, to the measure of the stature which belongs to the fullness of Christ.
These responsibilities are critical for the life and health of the church. It is clear that an elder must be spiritually astute. He must be faithful in his own spiritual disciplines so that he can instruct others in the faith. He must be able to recognize theological danger and address it. He must be actively sharing his faith and discipling others, including those who will be the future leaders of the church.
It is dangerously easy to assume that elders are a group of men that meet behind closed doors to carry on the work of the church. While they will certainly have decision-making responsibilities, they are primarily focused on shepherding the flock and protecting them from danger.
Paul reminds Timothy that if he is to serve well, he must be careful about his life and teaching.
Pay close attention to yourself and to your teaching; persevere in these things, for as you do this you will ensure salvation both for yourself and for those who hear you.
1 Timothy 4:16