If you ask enough people who follow other religions or claim to be atheists why they don’t believe in Jesus, you’ll find there are just about as many reasons as there are people to ask. If you investigate enough, you’ll also find that the reason (or reasons) don’t really matter much to a person who is looking for a reason not to believe. The truth is that people who choose not to believe do it because of evil desires in their hearts. It’s just that simple. So, answer their questions if you can (and if they have any), and live out your faith before them, but know that it is the Lord’s work to break through to their hearts. All the great answers in the world won’t be enough for someone who wants to be their own god.
“This is the verdict: Light has come into the world, but men loved darkness instead of light because their deeds were evil.” John 3:19