Think not that I come to abolish the law, or the prophets. I am come not to abolish, but to fulfill. For amen, I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the the law, till all be accomplished. Whosoever therefore shall disregard one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven. But whosoever shall do and teach them, the same shall be called great in the kingdom of heaven. For I say unto you, That except your righteousness shall exceed the righteousness of the scribes and Pharisees, ye shall in no case enter into the kingdom of heaven. (Matthew 5:17-20)
It is not possible that Jesus Christ could oppose God, or His laws, because He could not oppose Himself. God laid out the Ten Commandments, which were quite impossible to follow to the letter of the law, and then Christ-God led a perfect, sinless life and thus fulfilled the law. In living that kind of perfect life He brought God’s grace to us so that we might, through Him, accomplish those moral and spiritual perfections that we could not accomplish on our own. St. John Chrysostom saw a number of intersecting themes in this fulfillment: "Christ then fulfilled the Prophets by accomplishing what was therein foretold concerning Himself – and the Law, first, by transgressing none of its precepts; secondly, by justifying by faith, which the Law could not do by the letter," he wrote. Christ spoke about the coming fulfillment of the law because he was about to lay out a new code of conduct for the people of God. Also, as the Blessed Theophylact explains in The Explanation of the Holy Gospel According to Matthew, Christ fulfilled the law (as it says in the well-known King James Version passage, to more thoroughly flesh out God’s intention for His people. "Whatever the law had sketched in outline, Christ fully painted in. The law said, ‘Do not murder,’ but Christ said, ‘Neither be angry without a cause.’ So too the painter does not destroy the sketch, but rather completes it."
The next line has to do with when this fulfillment is going to happen. "For amen (verily) I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be accomplished (fulfilled)" — strikes a profound chord on both a micro and on a macro level. For one, it refers to the passing of the heaven and earth. This does not mean the law shall not change until the end of heaven and earth (that’s the common misunderstanding among many evangelical Christians), but it implies that heaven and earth will change — that transformation was due very shortly when something would be fulfilled or accomplished. Then the law would be changed. Now, what is being fulfilled is indicated by the other half of this passage" one jot or one tittle" — referring to the most minuscule of the Hebrew characters. The letter "jot" is most closely represented by Greek iota (an i) while the tittle is an accent mark. These two marks, combined, compose a cross. Therefore, one reading of this passage, according to the Holy Orthodox Church, is that the law will be fully accomplished by the cross (on which Christ would be crucified), at which time heaven and earth would be transformed into a new reality. This perhaps is one of the most Christological statements in the Holy Scriptures. When Christ is put on the cross, "the world passes away and undergoes a change in form," writes the Blessed Theophylact. "He is saying, therefore, that while the universe subsists not the least letter of the law will pass away." Therefore, Christ’s death on the cross marked the end of an age, and as the temple veil tore asunder, and then days later, the Resurrected Christ appeared, a new age had already begun. "Some say that the ‘jot’ and the ‘tittle’ signify the ten commandments of the law; others say that they indicate the Cross, for the iota is the upright beam of the Cross, and the accent, the transverse beam," the Blessed Theophylact adds.
The next passage tells us just how critical it is for the Christian to obey these new commands — but he packages them softly. "Whosoever therefore shall disregard one of these least commandments…" The "least commandments" refer to the new guidelines that Jesus was about to set forth (no anger without a cause, no lusting after women in your heart, giving to those in need, loving your enemies, etc) are "these least commandments." "He calls them ‘least’ out of humility, to instruct you, O reader, to have moderate thoughts of yourself as you give your teachings." But then he uses "least again," but the object of this usage of the word is quite different. "He who ‘shall be called least in the kingdom of heaven’ means he who will be last in the resurrection and who will be cast into gehenna," continues the Blessed Theophylact. He follows this with a call not only to follow the new commandments, but to teach them too, "for how can I guide another along a road that I have not myself travelled," the Explanation explains. In other words, "do as I say not as I do" has no place in a Christian context.
Lastly, the passage "Except your righteousness shall exceed the righteousness of the scribes and Pharisees, ye shall in now way enter into the kingdom of heaven" basically tells us that following the law is not enough to enter into the kingdom of heaven. We must surpass that — by following the addenda that Jesus next puts on the commandments. (Thou shalt not only not kill, but shall not even be angry at your brother, or call him names, or a fool, etc. Thou shalt not only not commit adultery, but thou shalt not be driven by mental lusts either.) Jesus, says the Blessed Theophylact, "teaches us us how we can exceed the righteousness of the scribes and Pharisees, and He enumerates the virtues" next.
And of course, the new law of God was grace, which becomes the only means of true righteousness. It’s as if God through the Ten Commandments had laid down a primer course for souls seeking righteousness, and then once one attains that, he must go deeper and the way to accomplish that was through adherence to the Living Word of Christ. "By righteousness is here meant universal virtue. But observe the superior power of grace, in that He requires of His disciples who were yet uninstructed to be better than those who were masters unto the Old Testament. Thus He does not call the Scribes and Pharisees unrighteous, but speaks of ‘their righteousness.’ And see how ever herein He confirms the Old Testament that He compares it with the New, for the greater and the less are always of the same kind," said St. John Chrysostom. The superiority of the teaching of Christ is evident in this Sermon on the Mount, and so is the utter impossibility of attaining this level of righteousness without God’s grace.