I noticed something today in studying Matt 18:6-14. I completely agree with Bob Wilkin in his book The Ten Most Misunderstood Words in the Bible (p. 68) that this section is about believers. I was really struck with the similarity of Matt 18:14 and John 6:39. Each of those verses speak of the will of the Father and each say that it is His will that no believer should perish (apollumi):

Even so it is not the will of your Father who is in heaven that one of these little ones should perish [apollumi] (Matt 18:14).

This is the will of the Father who sent Me, that of all He has given Me I should lose [apollumi] nothing, but should raise it up at the last day (John 6:39).

Also, John 10:27-29 says basically the same thing without using the exact expression the will of the Father:

My sheep hear My voice, and I know them, and they follow Me. And I give them eternal life, and they shall never perish [apollumi]; neither shall anyone snatch them out of My hand. My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of My Father’s hand.