Some Were Doubtful
In this passage, do we see only the eleven disciples or many more people, and if so, who were these other people? After all, who were the ones who doubted?
Refer back to Jesus appearing to the disciples (with Thomas present) where he removes all doubt.
"Then He said to Thomas, “Reach here with your finger, and see My hands; and reach here your hand and put it into My side; and do not be unbelieving, but believing.” - John 20:27
So who was doubtful at the Great Commission? Maybe they were doubtful they could accomplish the Great Commission, but I suspect that more people were present on this mountaintop. Many more.
Let's bring in 1 Corinthians 15: 3-8. This is the only reference to Jesus appearing to more that 500 brethren at one time. Could it be that any doubters would come from this crowd?
Barton writes about doubt: No Christian grows in faith without some doubt. The five-year-olds who took in every Bible story will become the fifteen-year-olds who want to know how, what, why, when, and where. And they will grow, too, and press for deeper answers along the way.
When you doubt, don’t be discouraged. It’s not a sin nor a failure. It’s a normal part of spiritual growth. Keep talking with thoughtful Christian friends and teachers, keep studying and praying, keep serving the Lord, and keep asking questions and looking for answers. God gave you a mind to discover his truth. Don’t let anyone tell you that discovery is wrong.
Matthew 28:17New American Standard Bible: 1995 Update
When they saw Him, they worshiped Him; but some were doubtful.