“Don't be in a hurry to condemn because he doesn't do what you do or think as you think or as fast. There was a time when you didn't know what you know today.”
What did he mean by "There was a time when you didn't know what you know today”?
"Don't be in a hurry to condemn"
Before criticizing somebody,
"...because he doesn't do what you do or think as you"
understand that that person did not have the same experiences or opportunities you did, in order to think the way you do.
"...or as fast."
Compared to you, he could potentially be not as smart.
"There was a time when you didn't know what you know today."
You did not come out from your mother's womb knowing everything. You took time to learn what you know today. You may not have known what you know now the week before.
Essentially, Malcolm X is saying live and let live which by definition stands to mean:
You should tolerate the opinions and behaviour of others so that they will similarly tolerate your own.
Think of learning to ride a bike. At one point you couldn't. Then you learned how. Now you can. You now know more than you previously did. You've learned. You're always learning, as you grow older. You'll never know THEN what you know NOW!! It's all in the same category talking about human growth and change.