Meaning, Muhammad (saws) has come to you with Islam, the religion which Allah (swt) decreed for His servants. Therefore, believe in what he has brought you and follow him, for this is better for you.

Otherwise, if you deny his Message and what he brought to you from your Lord, then your denial of that will not harm others but you. That is because Allah (swt), has all that is in the heavens and the earth, the possession and creating are for Him (swt), and your disbelief in what you disbelieved in His commands does not diminish in anything, and your disobedience would cause no diminution in His dominion and authority.