Al-Baqara 2:28

Allah testifies to the fact that He exists and that He is the Creator and the Sustainer Who has full authority over His servants,

كَيْفَ تَكْفُرُونَ بِاللَّهِ

(How can you disbelieve in Allah)

How can anyone deny Allah's existence or worship others with Him while;

وَكُنتُمْ أَمْوَتًا فَأَحْيَـكُمْ

(You were dead and He gave you life) meaning, He brought them from the state of non-existence to life. Similarly, Allah said,

أَمْ خُلِقُواْ مِنْ غَيْرِ شَىْءٍ أَمْ هُمُ الْخَـلِقُونَ - أَمْ خَلَقُواْ السَّمَـوَتِ وَالاٌّرْضَ بَل لاَّ يُوقِنُونَ

(Were they created by nothing Or were they themselves the creators Or did they create the heavens and the earth Nay, but they have no firm belief) (52:35-36) and,

هَلْ أَتَى عَلَى الإِنسَـنِ حِينٌ مِّنَ الدَّهْرِ لَمْ يَكُن شَيْئاً مَّذْكُوراً

(Has there not been over man a period of time, when he was not a thing worth mentioning) (76:1).

There are many other Ayat on this subject. Ibn Jarir reported from `Ata' that Ibn `Abbas said that,

وَكُنتُمْ أَمْوَتًا فَأَحْيَـكُمْ

(Seeing that you were dead and He gave you life) means, "You did not exist beforehand. You were nothing until Allah created you; He will bring death to you and then bring you back to life during Resurrection." Ibn `Abbas then said, "This is similar to Allah's statement;

قَالُواْ رَبَّنَآ أَمَتَّنَا اثْنَتَيْنِ وَأَحْيَيْتَنَا اثْنَتَيْنِ

(They will say: "Our Lord! You have made us to die twice and You have given us life twice.") (40:11)"

Tafsir Ibn Kathir (abridged, English).