That's why we should remain a part of his body through a continued believing. For the passage also says if we deny him (which causes us to not be a part of his body) he no longer has any obligation to continue to remain faithful to us.
"If we deny Him, He also will deny us" - 2 Timothy 2:12
"if the unbelieving one leaves, let him leave " - 1 Corinthians 7:15
God doesnt have any "obligation" but He still does..He is still faithful to us even when we arent faithful to Him..question..how many of US abide by beinf faithful ALL the time to God..giving him the FIRST of everything in our lives..time..money..energy..love..etc..how often does "life get in the way" and whoops..there goes prayer and devotions..or an nexpected bill come up and whoops..there goes my tithe and offering etc? IVE been guilty of that but God remains faithful to me..even when i sin.God doesnt like it..but Hes no less faithful..