These two phrases mean basically the same thing. John uses them to emphasize that God will surely forgive our sins. AT: "and will completely forgive us of what we have done wrong." (See: [[rc://en/ta/man/translate/figs-parallelism]])
"it is the same as calling him a liar, because he said we have all sinned." The phrase "because he said we have all sinned" is implicit. (See: [[rc://en/ta/man/translate/figs-explicit]])
## his word is not in us ##
This can be translated "we do not understand or obey what he says."