Islam welcomes questions. The Quran quotes the doubts of people, and its answer is to reflect, to look for evidence and to be honest. It also says that guidance is in God's hand and that no one can be forced to believe.
The fuller answer
Even the Prophet Abraham, in the Quran, asked God to show him how He gives life to the dead, in order that his heart might be at ease, and God did not rebuke him. Doubt, when it leads to seeking, is respected. Muslims are told to respond to disbelievers with wisdom and good words, and to say 'to you your religion, and to me mine'.
A Muslim would invite a sceptic to consider the evidence for God, to read the Quran with an open mind and to ask for guidance. Beyond this, a Muslim does not have the right to judge anyone's heart or to treat them with contempt.
And [mention] when Abraham said, "My Lord, show me how You give life to the dead." [Allah] said, "Have you not believed?" He said, "Yes, but [I ask] only that my heart may be satisfied." [Allah] said, "Take four birds and commit them to yourself. Then [after slaughtering them] put on each hill a portion of them; then call them - they will come [flying] to you in haste. And know that Allah is Exalted in Might and Wise."
Invite to the way of your Lord with wisdom and good instruction, and argue with them in a way that is best. Indeed, your Lord is most knowing of who has strayed from His way, and He is most knowing of who is [rightly] guided.
This answer follows the mainstream Sunni understanding and is general learning, not a fatwa. Where Muslims differ, the answer says so. Have a question of your own? Write to us.