The word common has many meanings, among them
- belonging to or shared by two or more people ⇒ common property
- belonging to or shared by members of one or more nations or communities; public ⇒ a common culture
- of ordinary standard; average ⇒ common decency
- prevailing; widespread ⇒ common opinion
- widely known or frequently encountered; ordinary ⇒ a common brand of soap
- widely known and notorious ⇒ a common nuisance
- (derogatory) considered by the speaker to be low-class, vulgar, or coarse ⇒ a common accent
- (prenominal) having no special distinction, rank, or status ⇒ the common man
While the phrase as used in the quote appears to suggest meaning 8 (common humanity analogous to the phrase common man), the context indicates that it is actually being used contemptuously, as in meaning 7.
The speaker holds himself in advance of common humanity, that is superior to, or more evolved, having achieved a spiritual plane.