“For what thanks can we render to God again for you, for all the joy wherewith we joy for your sakes before our God; night and day praying exceedingly that we might see your face, and might perfect that which is lacking in your faith?” 1st Thessalonians 3:9-10

Every born again child of God is given “the measure of faith” in the new birth (Romans 12:3). This seed of faith makes trusting in God and believing the gospel possible. This measure of faith is the same for every child of God. It is also a permanent and constant part of the spirit of the regenerated person. Since this faith is God-given in the new birth, and is also permanent and constant, this is not the faith which Paul seeks to perfect. This faith is already perfect.

But there is another aspect of faith that varies from person to person and does require gospel instruction to grow and mature. Gospel faith uses the measure of faith given in the new birth to comprehend and rejoice in the truth of God’s righteousness and salvation.

This gospel faith is fed by another “faith that was once delivered to the saints.”(Jude 1:3) During the ministry of Paul and the other Apostles during the first century, God delivered to the church the doctrines and practices that should be taught until the Lord returns.

This “faith once delivered to the saints” is what Paul has in mind here. He desired to finish laying the foundation of doctrinal principles that every child of God needs to understand about God and His sovereign work in salvation.