As for the gathering grounds on the Day of Resurrection, they will be tested there, just as they are tested in the barzakh (the intermediate state). One of them is asked:

“Who is your Lord? What is your religion? Who is your prophet?”

Allah Most High says in the verse:

“On the Day when a shin will be uncovered and they will be called to prostrate, but will not be able to.”

The hadith concerning Allah manifesting Himself to His servants at the gathering has been established through several chains of transmission in the authentic hadith collections. It relates what happens when it is said:

“‘Let every people follow what they used to worship.’ The polytheists follow their gods, while the believers remain. Then the Lord manifests Himself to them in a form other than the one they recognize, and they do not recognize Him. Then He manifests Himself to them in the form they recognize, and the believers prostrate to Him. The hypocrites’ backs, however, remain stiff like cattle’s horns: they wish to prostrate but cannot. He also mentioned His words in the verse: ‘On the Day when a shin will be uncovered and they will be called to prostrate, but will not be able to.’”

These matters are discussed at length elsewhere. And Allah knows best.