Here John probably means that believers' ability to love God and each other sincerely (See: [1 John 3:18](./16.md)) is a sign that their new life has indeed originated from the truth about Christ.
Here "hearts" is a metonym for people's thoughts or consciences. Here "hearts condemn us" is a metaphor for feeling guilty. AT: "if we know that we have sinned and as a result feel guilty" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-metaphor]])
Here "hearts" is metonym for people's thoughts or consciences. For God to be "greater than our hearts" means that God knows more than a person. Therefore he can judge things better than a person can. The effect of this truth is probably that God is more merciful than our consciences would suppose. AT: "God knows more than we do" (See: [[rc://en/ta/man/translate/figs-metonymy]])
God's opinion is spoken of as if it depends on what he sees happen in front of himself. AT: "we do what pleases him" (See: [[rc://en/ta/man/translate/figs-metaphor]])