To call someone a believer now may rely on present profession and available indications. To guarantee that person's ultimate success is a different claim requiring knowledge of their final condition.
Someone may spend much of life in wrongdoing or disbelief and later repent; conversely a person may outwardly practice for years yet subsequently abandon faith.
Distinguishing present status from final outcome avoids both presumptuous certainty based on earlier conduct and the unjustified denial of the possibility of later repentance.