The core doctrines of the gospel are simply put to the reader by John in terms anyone even child can understand. From these core points we can expand the doctrine outward. The over arching truth is that any notion contrary to these points is either a lie by us or else a lie by Him; there is no middle ground. Light, fellowship with the Light, fellowship with each other, sin, confession of sin, forgiveness of sin and the cleansing of sin from unrighteousness are all a one way or no way proposition. Who shall be the liar here; man or Christ?
There is so much said here that entire books can be (and have been) written. For the moment it should be enough to consider that these things are all expansions of the core doctrine that John presented in kjv@1John:1 , namely that God is light and in Him is no darkness. If we are in God, so too there can be no darkness in us. Knowing the 'how' this is possible is knowing the 'what' Jesus accomplished in His death and resurrection and the 'who' He is. The working of this knowledge produces unfeigned love in us for the brethren, which is the proof positive of possessing this knowledge. It can be produced in no other way.
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