When there's an earthquake, big buildings sometimes collapse while other buildings stand firm. They go through identical tremors but the response is quite different. We talk about standing firm because we should make sure that when the inevitable tremors of life come our way, our faith does not collapse -- that it should stand firm at that time.
Let me give you three reasons why this is important. First of all because faith has an eternal orientation. Some people come to faith initially because they have an immediate problem for which they see no solution and someone tells them that God can address that problem and bring them help and encouragement. They begin to understand, however, that God in His grace not only deals with the immediacy of their problem but He also gets into their past and begins to deal with all that has gone wrong prior to that problem, and He brings them forgiveness. If they begin to understand their faith properly they will realize that in addition to the immediate problem and the past, there is the whole of their future in which they are to live in the faith. There is a future, an eternal dimension to their faith.
Paul talks about this in 1 Thessalonians 2:13
when he reminds the Thessalonians of the day that the Lord Jesus will come again with all His holy ones. Over and over again in this first Thessalonian epistle there are statements concerning the return of the Lord Jesus. Our faith not only deals with our past, our immediate problems, and the foreseeable future but also is rooted in the return and ultimate victory of Jesus Christ as King of Kings and Lord of Lords. Those who continue to the end are those who are truly anticipating the return of the Lord and are strong in their faith.
The second reason that standing firm in the faith is important is that faith and faithfulness are used interchangeably in the New Testament. The reason for that is there's only one Greek word for both of them. The Greek word pistis is the word for faith and it is the word for faithfulness. The assumption in the Greek is that if you have faith you'll be faithful. To the extent that you're full of faith you will be faithful; those who are faithful are those who go on being full of faith.
Now this isn't a word game because we understand that if we begin to recognize our faith is in a faithful God, and He proves Himself worthy of our faith, then there is a reasonable assumption on His part that we -- having discovered His faithfulness -- will be faithful to Him.
Paul is particularly concerned about the Thessalonians. They are very young believers and they're under intense persecution, so he sends Timothy to them. He is quite specific: he sends Timothy to strengthen and encourage them in the faith and to find out how they are doing in the faith. Why? Because he wants to make sure they are continuing faithful to the Lord Jesus.
There are many instances of this kind of emphasis in the Scriptures. For instance, writing in the first chapter of Colossians 1:22
Paul says "He has reconciled you by Christ's physical body through death to present you holy in His sight without blemish and free from accusation." In the second chapter he encourages us to receive Christ, continue to live in Him, rooted and built up in Him, strengthened in the faith and overflowing with thankfulness.