It is focused on the danger posed to the church by those inside the church who are offering false teaching and causing divisions. (Having reached the end of the epistles, it might be interesting to consider which is the greater threat to early Christians -- persecution from non-Christians or questionable teaching from Christians.) Jude warns that such unfaithful and unbelieving teachers will receive God's judgment, just as other nonbelievers have been punished by God in the past.
Interestingly, though, while Jude warns Christians not to follow after these "grumblers and malcontents," he also instructs them to still rely on the love and forgiveness from God that are the great virtues of the church. By such love and forgiveness, these false teachers might yet be brought to true faith in Jesus Christ. Regardless, judgment is God's, not the church's.