Are the teachings on the mount part of the Old Covenant or of the New Covenant? Or is it mixed?
If they are part of the Old Covenant, how could Jesus teach what is the opposite to what was written in it? (See A.)
If they are part of the New Covenant, why do we need to forgive others to be forgiven by God? (See B.)
If it is mixed, how to know what is belonging to which covenant?
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A. "You have heard that it was said, 'Love your neighbor and hate your enemy.' But I tell you, love your enemies..."
Mt 5:43
B. "But if you do not forgive others their sins, your Father will not forgive your sins."
Mt 6:15
I think you miss the point Jesus was making in Matt. 5.
Jesus just got through saying that the Law would be here as long as heaven and earth. Then he says.
Matt. 5:
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.
Why would He say this? Because the Pharisees, the mainstream preachers of His time, were teaching for doctrines the Commandments of man, and not from God.
He went on to say:
Matt. 5:
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:
But the Law and Prophets taught more than that.
Lev. 19:
17 Thou shalt not hate thy brother in thine heart: thou shalt in any wise rebuke thy neighbour, and not suffer sin upon him.
Jesus was preaching against the teachers and doctrines of His time and before, that killed the Prophets. He isn't changing or preaching against His own Word's that He said before coming to earth as a man, or God's Commandments given to man through Moses.
Matt. 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.
The New Covenant had nothing to do with the definition of God's Laws, but rather the "Priesthood", or more precisely:
#1. How God's Commandments are administered. (No more Levite scribes to go to, to hear the Word's of God, Jesus writes them on our hearts.)
#2. the manner in which sins are forgiven. (No more Levitical Priesthood "works of the Law" for remission of sins. Jesus, with His Own Blood, cleanses our sins)