Therefore, since we have so great a cloud of witnesses surrounding us, let us also [like them]…run with endurance the race that is set before us (12:1)
We can trust God because of the testimony of those who have trusted Him before us!
When you read the chapter before this verse, you see that the “great cloud of witnesses” is talking about those who have stepped out in faith and trusted God in the past. In every case, when they stepped out in faith, God was there, meeting them at their point of trust, accomplishing His work through them, often in supernatural ways.
Why can we trust God? Because He has never failed anybody yet. Ever.
It is true that God does not always do what we want. It is only trust if we allow God to answer our prayers and faith according to His insight and wisdom and purpose.
If your faith is weak and you are struggling to trust God, I encourage you to read some of the great biographies of Christians who have taken great steps of faith and found God faithful. Read Bruchko by missionary Bruce Olson, or read about George Muller or Hudson Taylor. Or read in your Bible about Daniel or Joseph or the Apostle Paul. Your faith will be strengthened.