Therefore I tell you, whatever you ask in prayer, believe that you have received it, and it will be yours (Mk 11:24).
The Gospels contain several sweeping promises like this. Liberals don't think Jesus spoke most of the words attributed to him in the Gospels. Rather, they think anonymous authors, writing about two generations later, who had no personal or reliable knowledge of the historical Jesus, inventing sayings which they put on Jesus' lips. They think the Gospels reflect the viewpoint of the church, not the viewpoint of Jesus. Reflect the outlook of the time when they were written rather than the outlook of Jesus' time. Essentially, the Gospels are a vehicle to backdate later developments.
Let's play along with that contention for the sake of argument. Why would a writer invent these sweeping promises? In his own experience, and the experience of his fellow Christians, God didn't always grant their prayer requests. Indeed, one must ask if God usually grants prayer requests. So promises like this don't reflect the experience of "the church." Indeed, they generate a tension between the prima facie scope of the promise and the disappointing reality, which falls far short.
So why would Gospel narrators put these words in Jesus' mouth? It doesn't fit the theory of their late composition.