The apostles understood this. Back in the book of Acts, chapter 4, we come across an occasion in which Peter and John were used to heal a man who had been infirm for over forty years. Of course, that excited a crowd of onlookers, because this man was well known. Peter then took the opportunity of a crowd gathering to preach about Jesus Christ. What he said here is important in regard to the understanding of the importance of Jesus Christ and His sacrifice to you and me.
That phrase, "by which we must," means that it is absolutely obligatory.
