How can we reconcile a Hadith that promises entry into Paradise for whoever testifies that there is no god but Allah and that Muhammad is the Messenger of Allah, "without a doubt," with another Hadith that states, "Doubt is pure faith"?

Certainty (yaqin) is a prerequisite for the soundness of faith and the acceptance of the word of Tawhid (monotheism), in accordance with the saying of the Prophet, peace and blessings be upon him: "Whoever you meet behind this wall testifying that there is no god but Allah, certain of it in his heart, then give him glad tidings of Paradise," and His, the Almighty's, saying: "Indeed, the believers are those who have believed in Allah and His Messenger and then have not doubted." As for the statement that doubt is pure faith, it is incorrect. The correct understanding is that striving against satanic whispers and repelling them is from pure faith, as narrated in the hadith of Abu Hurayrah: "Some people from the Companions of the Prophet, peace and blessings be upon him, came and asked him, 'Indeed, we find in ourselves thoughts that one of us would deem too great to speak of.' He said, 'Have you truly found it?' They said, 'Yes.' He said, 'That is pure faith.'"