Yes, it was slavery. Much has been written about this topic, and the consensus is that slavery was the principal cause of the Civil War. There's a popular video going around right now that gives you a simple, five-minute explanation of the topic from the head of the history department at West Point. As the video states, there were other contributing causes, but slavery was far and away the leading cause for the outbreak of war.

It should be noted that we don't have the original copy of the Alexander Stephens speech quoted at length in the video. The best copy known was printed in the Savannah Republican five days after the speech and is not a precise transcription. In 1886, the Republican's copy was reprinted in a collection of speeches regarding the war. You can read two different versions of it here and here.

Regardless of the accuracy of the precise details, the speech's message is the same: Slavery was the cornerstone of secession.