The fulfillment of the Law in Christ means that his disciples must obey all of his commands (see Matthew 28:20).
commandments: This refers to Old Testament commands (5:18) now fulfilled in Christ's teachings (see 5:21–48).
least in the kingdom of heaven: Jesus may mean that such a person will still enter the kingdom, but only barely and with low status. Another view, based on some Jewish evidence, is that he means eternal exclusion or damnation (compare 8:12, where “those for whom the Kingdom was prepared” are shut out forever).