As Christians, we are to be caring for one another. We are all in a battle with the world, the flesh and the devil. And there are times when Christians fall into deliverate sin. Trespasses are serious sins. Paul is talking to Christians here. He says, "Brethren, …". This should make it pretty clear that it is possible for Christians to sin seriously (most of us realise that, but there are extreme positions which claim sinlessness is normal for Christians).
Serious sin can have devastating consequences for Christians, to the point that they could truthfully be described as "overtaken". That means that the sin now has the upper hand in their lives. And if that happens, it is important that those who are spiritual – that is, those who are walking with Christ, who are in Christ, who are filled and led by the Holy Spirit – it is important that such people EXIST in the church and secondly that they RESTORE those overtaken in sin.
This is an important ministry that is very needed in the church. Some very precious servants of God have been brought down by some kind of sin at various times in their lives. Remember, we are seeking to DO GOOD. We are never seeking to rejoice in evil, to be glad about the fallen state of another brother, as if that will somehow make us look more righteous by comparison. NO, we are commanded to RESTORE such a one.
And how are we to do it? The Bible says: "In a spirit of gentleness". That means, we don't do it harshly or with a judgemental spirit. We need to realise that this is the way to really help people. People who are fallen don't need harsh judgements from us. They need to be lifted up, and urged to turn back to God. If we are harsh, critical and judgemental many times our pride will open the door for our own fall! It is better NOT to be tempted. Jesus said, "Lead us not into temptation…". I would rather never be tempted by evil, if that were possible. At least, let such incidents be at an absolute minimum. Temptation is dangerous. What we want is to live in a healthy fear of God, hating evil, and reckoning ourselves dead to sin.
So if you see a brother fallen and overtaken by sin, if you are a spiritual person, lift the brother up.