No he was not in doubt. He was sure but he wanted to move to the level of certainty. That is why he said, “Show me how“, and he did not say, “Show me If you can”.
(And ˹remember˺ when Abraham said, “My Lord! Show me how you give life to the dead.” Allah responded, “Do you not believe?” Abraham replied, “Yes I do, but just so my heart can be reassured.” Allah said, “Then bring four birds, train them to come to you, ˹then cut them into pieces,˺ and scatter them on different hilltops. Then call them back, they will fly to you in haste. And ˹so you will˺ know that Allah is Almighty, All-Wise.”) Al-Baqara 2:260
بقوله “كيف” واضح بأنه كلام متيقن لكن أراد المزيد من اليقين. فهو أراد أن ينتقل من مرحلة علم اليقين إلى مرحلة عين اليقين. لذلك قال، ” أرني كيف” و لم يقل “إذا”
وإذ قال إبراهيم ربي أرني كيف تحي الموتى قَالَ أَوَلَمۡ تُؤۡمِنۖ قَالَ بَلَىٰ وَلَـٰكِن لِّيَطۡمَٮِٕنَّ قَلۡبِىۖ قَالَ فَخُذۡ أَرۡبَعَةً۬ مِّنَ ٱلطَّيۡرِ فَصُرۡهُنَّ إِلَيۡكَ ثُمَّ ٱجۡعَلۡ عَلَىٰ كُلِّ جَبَلٍ۬ مِّنۡہُنَّ جُزۡءً۬ا ثُمَّ ٱدۡعُهُنَّ يَأۡتِينَكَ سَعۡيً۬اۚ وَٱعۡلَمۡ أَنَّ ٱللَّهَ عَزِيزٌ حَكِيمٌ۬ ( البقرة 2 : 260