Be it with skepticism or faith in the Buddha’s teachings, the Buddha welcomes all to question them, all to question him.
Near the beginning of Śākyamuni Buddha’s life of extensive teaching, as recorded in the Kālāma Sutta, he encountered the Kālāmas of Kesaputta. To the Buddha, they reported that there were others who come visit them, who teach only their own teachings, while reviling others’ teachings. Thus, ‘there is doubt, there is uncertainty in us concerning them. Which of these… spoke the truth and which falsehood?‘
As translated by Soma Thera, the Buddha replied, ‘It is proper for you, Kālāmas, to doubt, to be uncertain; uncertainty has arisen in you about what is doubtful.’ Alternatively translated by Thanissaro Bhikkhu, ‘Of course you are uncertain, Kālāmas. Of course you are in doubt. When there are reasons for doubt, uncertainty is born.’ In short, the Buddha emphasised that it is right to doubt the doubtful.
Among other teachings, the Buddha then taught on how to assess the truthfulness and helpfulness of that learnt from anyone, including him. At the end, the Kālāmas marvelled at what was learnt, and requested for refuge through the Buddha. Just as it marked the beginning of the Kālāmas’ proper learning of the Buddha’s teachings, the Kālāma Sutta has since become a text to study for Buddhist beginners.
Near the end of the Buddha’s teaching life, as recorded in the Sūtra Of The Buddha’s Bequeathed Teachings《佛遗教经》, the Buddha said, ‘You [and] others, if of suffering [and] other [aspects of the] Four [Noble] Truths [i.e. which encompass all Dharma teachings], have those [with] doubts, [you] may swiftly ask them, [to] not have doubts, without seeking resolution.’ (汝等若于，苦等四谛，有所疑者，可疾问之，毋得怀疑，不求决也。)
At that time, [the] World-Honoured [One, as] such thrice called aloud, [and the] people [were] without questions. What [is] that [so]? [As the] assembly [was] without doubts thus. (尔时世尊，如是三唱，人无问者。所以者何？众无疑故。) Then, Aniruddha observed [the] assembly’s minds, and [to the] Buddha said, ‘… [It] is [so, that] all Bhikṣus, of [the] Four [Noble] Truths within, [are] definitely without doubts.’ (时阿㝹楼驼，观察众心，而白佛言：… ‘是诸比丘，于四谛中，决定无疑。’)
The Buddha thus urged active enquiry in learning the Dharma, from the beginning to the end. As such, we too should actively enquire when learning the Dharma, so as to learn it well, by resolving all doubts about it. To doubt and ask about the Dharma is not to slander, while to ignore one’s ignorance, and to deludedly speculate about it could lead to slander. Just as the Buddha’s welcoming and answering of the Kālāmas’ question became a teaching for them, our questioning too can lead to the truth.
Are we like the Kālāmas? Well, there are three kinds of people – those  who believe blindly in speculations of their own and others, without doubting or direct clarifying at all,  who have only some doubting and thus only some direct clarifying, and  who actively doubt and directly clarify. To clarify doubt, thus transforming it to understanding, is how the truth can be realised!
If the most worthy Buddha should not be asked about his teachings, by his present and future disciples, who should be asked about them?