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.
"Hearts" here is a metonym for "thoughts and intentions." AT: "if the way we think and the way we intend to act make us feel guilty" (See: [[rc://en/ta/man/translate/figs-metonymy]])
God knows more than a person and judges better. The effect of this truth is probably that God is more merciful than our consciences would suppose. This quality of God is expressed by the term "greater." 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]])