They alone are true, whose love for God is deep and heart-felt. Those who have one thing in their heart, and something else in their mouth, are judged to be false.
ਦਿਲਹੁ ਮੁਹਬਤਿ ਜਿੰਨੑ ਸੇਈ ਸਚਿਆ ॥ ਜਿਨੑ ਮਨਿ ਹੋਰੁ ਮੁਖਿ ਹੋਰੁ ਸਿ ਕਾਂਢੇ ਕਚਿਆ ॥੧॥
SGGS Ang 488
The same idea in other traditions
Keep thy tongue from evil, And thy lips from speaking guile.
O ye who believe! Fear Allah, and (always) say a word directed to the Right:
Lying lips are abomination to the Lord: but they that deal truly are his delight.
Him I call indeed a Brâhmana who utters true speech, instructive and free from harshness, so that he offend no one.
By simplicity he will become upright in actions, thoughts, and speech, and he will become veracious; thereby he will truly practise the Law.
The Master said, "Yu, shall I teach you what knowledge is? When you know a thing, to hold that you know it; and when you do not know a thing, to allow that you do not know it;—this is knowledge."
The true prevails, not the untrue; by the true the path is laid out, the way of the gods (devayânah), on which the old sages, satisfied in their desires, proceed to where there is that highest place of the True One.