But the Lord is faithful, and He will strengthen and protect you from the evil one.” (2 Thessalonians 3:3, NASB95)

Paul writes this in the context of his own experience of opposition from “perverse and evil men.” He well understood that any attempt to live faithfully as a disciple of Christ would lead to opposition and difficulty, and he knew the driving person behind the opposition: the evil one, i.e. Satan the opposer of God.

Paul knew something else however, he knew that “the Lord is faithful,” and this fact stood above all else.  Paul’s audience need not worry about “the evil one” because the Lord is faithful, it is he who will take responsibility to protect followers of Christ from the designs of Satan.  Paul’s logic goes something like this:  God is all-powerful and because he is all-powerful Satan is surely under his authority and power.  God is faithful–it is a fundamental part of his nature–therefore God will protect and strengthen and support and care for his own people, he can do no less.  In the battle then between Satan and God’s disciples, it will be God who is the deciding factor, his faithfulness ensures that he will care for his people and his power proves that he can do so, even in light of Satan’s plans.