Galatians 3:21-22 (ESV) – Is the law then contrary to the promises of God? Certainly not! For if a law had been given that could give life, then righteousness would indeed be by the law.  But the Scripture imprisoned everything under sin, so that the promise by faith in Jesus Christ might be given to those who believe.
Paul, having painted a picture of salvation by faith in Christ with the Law having nothing to contribute to salvation at all, supposes to know what an objection might be to his argument and puts it in the table to address it. This is a technique that he is fond of and uses often. See Romans 6:1; 9:14, 19.
The Gospel, indeed, all Scriptural truth, can stand the test of minute scrutiny. Do not be afraid to ask the hard questions that a true understanding of the Bible can produce. And do not run away from those non-believers who, in response to some teaching of the Scriptures, ask questions that are difficult. If the Bible is true, it can stand the inquiry. If it is not, then it should not. Paul was not afraid to put on the table, the questions that challenge the claims of the Gospel. Such questions need to be addressed with forthrightness and candour. They should not be avoided.
This necessitates that believers know their stuff. It means that we should treat real questions with respect. It means that we believe the Bible is true.
No one can answer all the objections that may arise from the various quarters where the Gospel is presented. When we cannot, we should just say so. The truth of a point is not determined by our ability to present it properly. But every believer should be a student of the Word of God and seek to know all the repercussions of his faith. Hebrews 5:11-6:2 should ring in our ears as an instruction to each one of us. So should II Timothy 2:15, I Corinthians 3:1-2; I Peter 2:1-2 and II Peter 3:14-18, Colossians 2:8 and Ephesians 4:11-16.
It is a shameful thing to believe something and not be able to defend it. It is even more shameful to do what so many in North America do with their faith and that is see no need to defend it. Too many believers relegate those who challenge the claims of Christianity with difficult questions to be unworthy of response. Ignorance of solid truth is disobedience.
Christian – think. And do the hard work that God calls us to in order to discover the great treasures that He has put in His Word.