Albert Barnes Commentary 1 Thessalonians 3:10

"night and day praying exceedingly that we may see your face, and may perfect that which is lacking in your faith?" — 1 Thessalonians 3:10 (ASV)

Night and day. Constantly.

Praying exceedingly. Greek: abundantly; that is, there was much more than ordinary prayer. He made this a special subject of prayer; he urged it with earnestness and without intermission. (Compare to 1 Thessalonians 2:17).

And might perfect that which is lacking in your faith. This means to render it complete, or fill up anything that is wanting. The word used here (katartisai) properly means to make fully ready, to put full in order, to make complete. (See Barnes on Romans 9:22; 2 Corinthians 13:9; Galatians 6:1).

It is rendered mending in Matthew 4:21 and Mark 1:19; perfect and perfected in Matthew 21:16, Luke 6:40, 2 Corinthians 13:11, 1 Thessalonians 3:10, Hebrews 13:21, and 1 Peter 5:10; fitted in Romans 9:22; perfectly joined together in 1 Corinthians 1:10; restore in Galatians 6:1; prepared in Hebrews 10:5; and framed in Hebrews 11:3. It is not elsewhere used in the New Testament.

The meaning here is, that whatever was deficient in their views of religious doctrine the apostle desired to supply. It is to be remembered that he was with them for only a comparatively short time before he was compelled to depart to Berea, and it is reasonable to suppose that there were many subjects on which he would have been glad to have an opportunity to instruct them more fully.