John Gill Commentary Deuteronomy 4:31

"for Jehovah thy God is a merciful God; he will not fail thee, neither destroy thee, nor forget the covenant of thy fathers which he sware unto them." — Deuteronomy 4:31 (ASV)

For the Lord your God is a merciful God
In Christ, in whom he has proclaimed his name as such, of which Moses had a comfortable view, (Exodus 34:6–8) and therefore could attest it from his own knowledge and experience:

he will not forsake you ;
though in a strange country, but bring them from there into their own land again, and favour them with his gracious presence in his house and ordinances:

neither destroy you ;
from being a people; and in a very wonderful manner are they preserved among the nations of the earth to this day:

nor forget the covenant of your fathers, which he swore unto them ;
that a Saviour should come and turn away ungodliness from them, and take away their sins; see (Romans 11:26Romans 11:27) .