Charles Ellicott Commentary Psalms 15:2

"He that walketh uprightly, and worketh righteousness, And speaketh truth in his heart;" — Psalms 15:2 (ASV)

Uprightly. —Literally, he whose walking is perfect rectitude. In Proverbs 28:18 the same phrase occurs. Compare to Isaiah 33:15.

Speaks the truth in his heart— i.e., both thinks and speaks the truth.

“This above all: to your own self be true,
And it must follow as the night the day,
You cannot then be false to any man.”

SHAKESPEARE: Hamlet.