Allah - there is no deity except Him. He will surely assemble you for [account on] the Day of Resurrection, about which there is no doubt. And who is more truthful than Allah in statement.
"There is no god but He": This is either a predicate for an implied subject (i.e., "He is Allah, there is no god but He"), or it is a parenthetical statement.
The predicate is: "He will surely gather you."
Its meaning: Allah—by Allah, He will surely gather you to the Day of Resurrection. That is, He will surely resurrect you unto Him.
"The Resurrection" (al-qiyāmah): It is like the word ṭullābah (pursuit) and ṭullāb (pursuers). It refers to their rising from their graves or their standing for the reckoning.
Allah the Exalted said: "The day when mankind will stand before the Lord of the worlds" (Al-Muṭaffifīn: 6).
"And who is more truthful than Allah in statement?"
Because He, the Almighty and Exalted, is truthful; lying is impossible for Him.
Lying is inherently something that deters one from committing it, due to its ugliness. The nature of its ugliness—which is being a lie and reporting something contrary to what it actually is—is such that no one lies unless they are compelled to do so to attain a benefit or ward off harm.
Or, they are independent of lying but are ignorant of their own independence, or they are ignorant of its ugliness, or they are foolish and do not distinguish between truth and falsehood in their speech, nor do they care which one they utter. Sometimes, lying is even sweeter to their palate than truth.
It is narrated that a fool was rebuked for lying, and he replied: "If you were to gargle your throat with it, you would never part with it."
It was said to a liar: "Have you ever told the truth?" He replied: "Were I not truthful in my saying 'no,' I would not have said it."
Thus, the Wise, the Self-Sufficient—to whom needs are impossible, who is All-Knowing of every known thing—is exalted above lying, just as He is exalted above all other ugly deeds.