This short letter is an encouragement in the faith and a warning against apostasy. It is a contrast between Christians and people who come into the church for church for other reasons.
Christians are, simply, faithful to the teachings of Christ and the apostles. This is both belief and action. They believe the truth and behave uprightly. Jude does not go into this as much as some of the other epistles.
Apostates, false teachers, and pretend Christians do not have these traits. They dress up other doctrines in Christian-sounding terms. Their behavior especially gives them away. They pursue their lusts, they speak ill of others, they are self-serving, and they are liars. And they will keep coming.
Of course, Christians aren’t perfect people. Believers should reflect on these things. Our salvation was always God’s work, as is our growth in Christ. However, we are to be reflective, self-monitoring, and humble. If we fall into bad ways and our conscience doesn’t prick us, if we aren’t moved to repent, we should consider our relationship with God.