The truth is spoken of as if it were an object that could be inside believers. AT: "we do not believe what God says is true" (See: [[rc://en/ta/man/translate/figs-metaphor]])
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 implied that a person who claims to be without sin would be calling God a liar since he said that everyone is a sinner. AT: "it is the same as calling him a liar, because he said we have all sinned" (See: [[rc://en/ta/man/translate/figs-explicit]])
"Word" is a metonym for "message." Obeying and honoring God's word is spoken of as if his word were inside the believers. AT: "we do not understand God's word nor obey what he says" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-metonymy]])