What exactly are the characteristics of a "true Christian"?

Impossible to determine. You're asking how to determine the exact characteristics of a nebulous concept defined partially but not wholly by a thousand-page bunch of poems and stories from 3rd and 4th hand accounts with contradictions and discrepancies. Asking for the true Christian definition is like asking what is the absolute purpose of the poem, except the poem is 10,000 times as long and much more complicated and written over thousands of years and judged in the thousands of years since then in an ever-evolving set of earthly cultures.