What Paul is saying here is that faith and the law are opposites and if a person chooses one he or she has in actual fact rejected the other. It is impossible to be saved by both works and faith at the same time. Putting it another way. If you are depending on the law to save you, you are depending on yourself, your abilities, your skills, your determination, your will power but if you are depending on faith in God, then you have turned your back on your will, skills, abilities and determination. Instead you are resting on God’s power to save you.