Then he addressed them: 'Men of Israel, consider carefully what you intend to do to these men. Some time ago Theudas appeared, claiming to be somebody, and about four hundred men rallied to him. He was killed, all his followers were dispersed, and it all came to nothing. After him, Judas the Galilean appeared in the days of the census and led a band of people in revolt. He too was killed, and all his followers were scattered. Therefore, in the present case I advise you: Leave these men alone! Let them go! For if their purpose or activity is of human origin, it will fail. But if it is from God, you will not be able to stop these men; you will only find yourselves fighting against God.'
Gamaliel is speaking to the Sanhedrin in this passage. This same Gamaliel was the one that taught religion to Saul (Paul). Now, he is trying to get the Sanhedrin to understand that if the apostles are not from God they will fail. If they are from God, then His will be done. We too can have this assurance from God. If we are doing His will, who can stop us?