The Cow - 2:209
And if you stray from the right path even though clear proofs have come to you, know that Allah is All-Superior, All-Wise.
The Qur'an contains clear proofs guiding to the Straight Path:
The Kneeling/45:20
These are enlightening proofs for the people, a guide and mercy for a people who have firm faith.
The Cattle/6:155
And this is a blessed Book which We have sent down. So you should follow it and act righteously so that mercy may be shown to you.
The Cattle/6:156
"So do not say, "Two groups before us were given the Book, but we were ignorant of their teachings"."
The Cattle/6:157
"Nor can you say, "If the Book had been sent down to us, we would have been on a more right path than them". There has come to you a proof from your Lord, a guidance and a mercy. Who is more unjust than he who denies the signs of Allah and turns away from them? We will punish those who turn away from Our Verses with the worst of torment for their turning away."
Originator/35:40
"Say, "Have you considered your partners whom you call besides Allah; show me what they have created in the earth?"Or do they have a partnership in the sky? Or have We given them a Book and they rely on a proof in it? Indeed, the wrong-doers give only deceitful promises to one another."