I was more interested in what systematic theology
@David H. was referring to. I din't know how that related to knowing when someone speaks falsely of you.
Each of us has a system and method that we come to understand God. Some use fundamentalism, others rely on experience, some rely on a manmade system of theology such as Calvinism or Wesleyanism. There are numerous systems that man can try to understand God, but he does not limit himself to one approach or the other to reveal himself to us. To limit this to your brand of systematic theology is to judge others as false. So your theology leads you to scripture, nothing wrong with that, But when you judge others because they do not conform to your system of theology then you are not in fellowship with your brethren and are denying the growth that comes from this fellowship. In other words, you will forever be stuck in the "we know in Part" because you are not making room for the Parts other believers have been revealed.
So for example, you deny the distinction between the faithful and saints, and in doing this, you are denying a deeper and fuller understanding of the who the saints are in Revelation (The book that mentions the saints the most), and how the church relates to the saints. Thus your system of theology closes the door to the revelation that comes from fellowship with other believers.
It is easy to fellowship with the likeminded for there is agreement there, but fellowship with those who are not likeminded is a true blessing once you recognize in humility we learn, in exhortation and rebuke and reproof we grow in our understanding. Fellowshipping and limiting yourself to the likeminded has led to the denominationalism we see in the Modern church and the warfare between these denominations. My rebuke of this mindset comes with an exhortation to fully employ Paul's "more excellent way", and that is the way of Love (1 Corinthians 13). This is what marks the church of the saints.