The truth about God is something that has progressively been given to people over time. Its kind of like that you dont get someone who has been dwelling in a cave in total darkness and then shine a flashlight in his face...
Prov:4:18: But the path of the just is as the shining light, that shineth more and more unto the perfect day.
NOTICE that the idea is that this new revelation Jesus is giving is showing us what the Father is truly like...
Matthew 5:
38: Ye have heard that it hath been said, An eye for an eye, and a tooth for a tooth:
39: But I say unto you, That ye resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also.
40: And if any man will sue thee at the law, and take away thy coat, let him have thy cloke also.
41: And whosoever shall compel thee to go a mile, go with him twain.
42: Give to him that asketh thee, and from him that would borrow of thee turn not thou away.
43: Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy.
44: But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
45: That ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust.
46: For if ye love them which love you, what reward have ye? do not even the publicans the same?
47: And if ye salute your brethren only, what do ye more than others? do not even the publicans so?
48: Be ye therefore perfect, even as your Father which is in heaven is perfect.
Now read this, Jesus did not come to destroy the law but to fulfill it, He came to DEMONSTRATE THE REALITY OF THE LAW. Isa:42:21: The LORD is well pleased for his righteousness' sake; he will magnify the law, and make it honourable:
Matthew 5:17:
Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil.
18: For 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 fulfilled.
19: Whosoever therefore shall break 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.
20: 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.
21: Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment:
22: But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire.
The Jews didnt understand the underlyhing foundation of the Law. It has always been Love God (first 4 commandments) and love your neighbor (last 6 commandments).
Mt:23:23: Woe unto you, scribes and Pharisees, hypocrites! for ye pay tithe of mint and anise and cummin, and have omitted the weightier matters of the law, judgment, mercy, and faith: these ought ye to have done, and not to leave the other undone.
Its not really that God has changed any. It has been a progressive revelation. Jms:1:17: Every good gift and every perfect gift is from above, and cometh down from the Father of lights, with whom is no variableness, neither shadow of turning.