Commenting on 1 Thessalonians 5:24, D. Edmond Hiebert says,

“God is faithful, to be trusted, reliable concerning all that He has said. … He never lies in making a promise and never begins a work without carrying it out to completion.”

The Thessalonian Epistles, p. 254.

That ought to encourage every believer. God will be with us to sanctify and preserve us. Why? He will do it because He is faithful to those He has called.