How in the world could the disciples remember all that the Lord said on the important occasion of that which we call the Sermon on the Mount? Have you ever memorized anything? I have and with much dread. Memorization is not pleasant to me; I had to do so in the school of preaching I attended. Just the same, there is value in it. Craig Blomberg remarks about the memorization approach of those who lived in the first century:
According to Riesenfeld, Jesus probably had his disciples memorize his most significant teaching and perhaps even certain narratives about what he did. Thus there is every reason to believe that they were reliably preserved…In addition to documenting how widespread and prodigious the practice of memorization was in ancient rabbinic circles (many rabbis had the entire Old Testament and much of the oral law committed to memory!)… (“The Historical Reliability of the Gospel,” Craig Blomberg, IVP, p. 25 (but especially see pages 25-31) 1987).
When one considers the educational approach of the rabbis, the Lord and His disciples had (coupled with the apostles being guided into all truth) I think it is perfectly reasonable to set aside any question concerning reliability.
Did some actually think Jesus was trying to abolish (destroy) the Law of Moses? This was an accusation leveled at not only Jesus, but the apostles also (Luke 4:16-30; Acts 6:14; 24:1-14). The idea of “abolish” or “destroy” is, in effect, to set aside as valueless. Jesus did not come to do this at all; rather, He came to fulfill. A world of difference there is between the two. Some people today still struggle with this idea; they think that God (Jesus) did not come to do away with the moral law as revealed in the Ten Commandments, but rather the Lord came to do away with the ceremonial law. Jesus did not make this kind of distinction, and neither did the apostles – this is strictly a man-made distinction. Jesus said the Law of Moses will pass away at a point in time; that point in time will be when all is fulfilled (vss. 17-18). God sent to this earth a man who could completely fulfill (bring to completion) all that the law stood for (cf. Matthew 22:34-40). He did not come to set aside the Law of Moses as valueless, because it had great value (cf. Galatians 3:24ff).