Argument.
The argument can be formulated as follows:
P1. If God exists, He is all merciful
P2. Many people do not follow God simply because they do not know about Him
P3. An all merciful God would not send people to hell for this
P4. God sends people to hell for this
C1. Therefore, God cannot be all merciful
C2. Therefore, God cannot exist
Response.
The hidden premise in this argument is that the only opportunity to hear about God is while they are alive. However, this is not the case and all are given the opportunity. From 1 Peter 4:6
For for this cause was the gospel preached also to them that are dead, that they might be judged according to men in the flesh, but live according to God in the spirit.
Therefore, the destiny of the unevangelized argument rests on a hidden premise which is not true.