If all prophets taught Islam, what was so different about Muhammad?
The teachings delivered by previous prophets had gradually become lost, altered and obscured. God then sent Muhammad to:
* re-establish those original principles,
* preach other truths which had not been preached before, but were necessary now because mankind had advanced,
* to collect together the best of every religion into one faith.
So Muhammad appeared as the Last Prophet to give to the whole world one, perfect and ever-lasting, religion - Islam.