  We present the discovery of a Type Ia supernova (SN) at redshift $z = 1.914$
from the CANDELS multi-cycle treasury program on the \textit{Hubble Space
Telescope (HST)}. This SN was discovered in the infrared using the Wide-Field
Camera 3, and it is the highest-redshift Type Ia SN yet observed. We classify
this object as a SN\,Ia by comparing its light curve and spectrum with those of
a large sample of Type Ia and core-collapse supernovae (SNe). Its apparent
magnitude is consistent with that expected from the $\Lambda$CDM concordance
cosmology. We discuss the use of spectral evidence for classification of $z >
1.5$ SNe\,Ia using {\it HST} grism simulations, finding that spectral data
alone can frequently rule out SNe\,II, but distinguishing between SNe\,Ia and
SNe\,Ib/c can require prohibitively long exposures. In such cases, a
quantitative analysis of the light curve may be necessary for classification.
Our photometric and spectroscopic classification methods can aid the
determination of SN rates and cosmological parameters from the full
high-redshift CANDELS SN sample.
