فَلَا تَكْتُمُنَّ اللَّهَ مَا فِي نُفُوسِكُمْ ۞ لِيَخْفَى وَمَهْمَا يُكْتَمِ اللَّهُ يَعْلَمِ
Meaning: Do not try to conceal from God what lies within your hearts; whatever is hidden, God knows it.
Fa-lā taktumunna Allāha warns against trying to conceal inner intentions from God. Mā fī nufūsikum distinguishes outward speech from inward motives: making an oath publicly is not enough when it is privately undermined.
In wa-mahmā yuktami Allāhu yaʿlam, mahmā broadens the statement to anything whatsoever that is concealed. The moral reasoning is that covert treachery may escape other people, but not divine knowledge. Keeping a covenant is therefore also a matter of conscience.