Tafsir of Al-Qasas 28:69

"And your Lord knows what their breasts conceal" — that is, what they harbor and hide within their breasts of false beliefs, their enmity toward the Messenger of Allah (may Allah exalt his mention and grant him peace), and the like.

"And what they declare" — and what they manifest of heinous deeds, criticism against him (upon him be prayer and peace), and the like.

Perhaps the expression "what their breasts conceal" is used for emphasis regarding the vileness of their inner state, because what is within them is the origin of the abominations that manifest externally. It did not say "what they conceal" (ma yakunnun), just as it said "what they declare" (ma yu‘linun).

Ibn Muhaysin read "takun" (تَكُن) with a fatha on the ta and a damma on the kaf.