‘But the Lord is faithful who will establish you and guard you from the Evil One (or ‘evil').
The faithfulness to His own of God and the Lord is Paul's constant theme (1Co 1:9; 1 Corinthians 10:13; 1 Thessalonians 5:24; 2 Timothy 2:13 compare also Hebrews 2:17; Heb 10:23; 1 Peter 4:19; 1 John 1:9). Behind all uncertainty we find the faithfulness of God. It is He Who watches over the Christian's life and will establish and protect us. To establish is to firmly found, to strengthen (compare 1 Corinthians 3:10-15). To build on the rock (Matthew 7:24-27).
Guarding from the ‘Evil One', and in view of chapter 2 this is the most likely emphasis rather than ‘evil' (compare the cry for deliverance in the Lord's prayer in Matthew 6:13), reminds us that God is over all and He protects His own from the machinations of Satan (Joh 17:15; 1 John 5:18; Romans 16:20). They are sealed by God (Revelation 7:3-4 with Revelation 9:4). Thus the evil men of 2 Thessalonians 3:3 are now seen as spurred on by the Evil One of 2 Thessalonians 3:4. Compare how in Revelation John can speak of persecuting Jews as ‘the synagogue of Satan' (Revelation 2:9; Revelation 3:9), and Pergamos, with its persecuting authorities, as the place of Satan's throne and where Satan dwells (Revelation 2:13). But the fact that we are guarded does not mean that we can be careless about the matter (Ephesians 6:10-19; 2 Corinthians 2:11). We must take heed to put on the armour of God.
Note the change from ‘us' to ‘you'. The memory of what he and his companions had to face also reminded him of the tribulation and persecution the Thessalonians were facing, so as he asked for their prayers, he also prayed for them. They were in partnership together and were to share each other's burdens.