I was really struck by the contrast between two passages in today's reading (which was Gen. 16-18 and Mark 6).
-"Is anything too hard for the Lord?" (Gen. 18:14)
-"And he could do no mighty work there, except that he laid his hands on a few sick people and healed them. And he marveled because of their unbelief." (Mark 6:5-6)
In the first instance, the angelic visitors reassured Abraham (and an eavesdropping Sarah) that God indeed could give the elderly couple a child, even though they would be 100 and 90 years old. But in the second instance, the text of Mark 6 says that Jesus could do very few miracles in Nazareth.
From one perspective, God's power is not limited by man's faith. God can do anything He wants to do. And even though Abraham and Sarah literally fell down laughing when God told them He intended to keep His promise and give them a son of their very own, He kept His word. So God's power is not inherently dependent on human faithfulness.
At the same time, there are some blessings made possible by divine power that God does choose to dispense on the condition of faith. Many of Jesus' miracles of healing fall into this category. Jesus told the woman He healed of the bleeding disease in Mark 5, "Your faith has made you well." His power was the basis of her healing; her faith was the means of her healing.
And that apparently is the reason Jesus could not perform many miracles in Nazareth. It wasn't as if His power suddenly vanished, or that He was thwarted by some evil force. He could not do many miracles there because there were not many believers there - a fact which amazed Him.
Salvation is another example of a mighty work of God that is contingent on our response. God did not have to make salvation conditional - He's God! But He has chosen to give us a choice - to allow us to accept or reject the work of redemption. And how He must marvel at the refusals of a dying world to accept eternal life through His Son.