Paul wants Timothy to find strength in the grace he has already received from Jesus. This strengthening is continual. Like taking vitamins, one does not take vitamins for a few days and then quit; you would just get weak and sickly again. It is the regular use of vitamins that build up some health and strength.
Why does Timothy need to be strengthened? He is suffering some sort of hardship, probably from within his churches. He is likely attacked for being too young, probably those who have defected from Paul see him as Paul’s deputy and therefore suspect, and he is possibly suffering some physical problems as well (“take a little wine for your stomach’s sake” might imply illness).
How does this strengthening happen? Paul gives no steps, but only states that it is “by grace in Christ Jesus.” This may seem a bit frustrating to the modern Christian since we would have preferred “ten steps to being strong in Jesus” at this point in the letter. Our relationship with Christ is not a series of hoops we have to jump through or achievement badges we earn. Our relationship with God in Jesus is more like a parent-child relationship. We do not start our at level one and work our way up to level ten, we are wholly a child of God from the moment we accept Christ as our savior.
Once again Timothy is simply told “be what you are,” a child of God. That status alone is the source of his strength – he can do all things through Christ (Phil 4:13).
In order to strengthen the whole church, Timothy is to pass along the things he has already heard from Paul to people who can be trusted to pass it along to a third generation. This is the chain of tradition we have already encountered in 2 Timothy. To whom is the tradition to be passed on? Paul calls the “faithful,” with the sense of trustworthy (sense of “competent, qualified and able”). By analogy, there are some people who you might think are reliable enough to house sit for you. If you get an untrustworthy person, then when you come home, your plants are dear and your pets are starved because they forgot to carry out their responsibility.
What is remarkable here is that Paul sets himself up as a standard, what Timothy heard from Paul in front of many reliable witnesses is to be passed along. This is like saying, make sure they know the standard Pauline sermon. That is foundational to everything else! This is not private teaching, or some sort of speculative teaching, but the sorts of things that Paul has always taught as truth and everyone knows is the core of the gospel.
The others who receive this tradition will teach in the future. Paul is not thinking of Timothy’s generation, but the people Timothy will disciple and prepare for ministry, they will be waning people that will be born long after Paul and Timothy are dead! This is really what all church work is about, preparing the next generation for serving Christ. Perhaps the reason that churches die is that they did not prepare the next generation to preserve to the Gospel.
Paul certainly would include lay leaders here, but looking ahead to the metaphors which follow, he has some kind of specialized training in mind. Not everyone is called to be a soldier nor should everyone train to be a soldier.
Just because you watched war movies does not qualify you to be an officer in the military. Sadly, many Christians think that watching YouTube sermons qualify them to be pastors, often leading to disaster!