رَبَّنَا لاَ تُزِغْ قُلُوبَنَا بَعْدَ إِذْ هَدَيْتَنَا وَهَبْ لَنَا مِن لَّدُنكَ رَحْمَةً إِنَّكَ أَنتَ الْوَهَّابُ

Rabbana la tuzigh quloobana ba'da idh hadaitana wa hab lana milladunka rahmah innaka antal Wahhab

Word by Word:

Rabbana: Our Lord

la: No

tuzigh: deviate

quloobana: our heart

ba'da: after

idh: when

hadaitana: YOU have guided us

wa: and

hab: grant

lana: us

milladunka: from Yourself

rahmah: mercy

innaka: indeed YOU 

anta: YOU are

Al-Wahab: The Bestower

Translation: Our Lord! (they say), Let not our hearts deviate now after Thou hast guided us, but grant us mercy from Thine own Presence; for Thou art the Grantor of bounties without measure [3:8]

Tafseer: The verse shows us that guidance and straying are from Allah alone. When Allah intends to guide someone, He makes his heart tilt towards what is good and right; and when He decides to let someone go astray, He turns his heart away from the straight path.

This is just as it was said in a hadith of the Holy Prophet SAWW there is no heart which is not there in between the two of Allah's fingers - He makes it firm on the truth as long as He wills, and turns it away from the truth when He wills.

He is Allah, absolute in power. He does what He wills. Therefore, those who are concerned about how to remain firm in their faith, they go to the source - requesting and praying Allah for steadfastness. The Holy Prophet SAWW always used to pray for it as it appears in a hadith:  O reverser of hearts, make our hearts firm on the faith chosen by You.