How do we know brotherly love is true?

According to 1 John 3:14, loving the brethren is an evidence of having passed from death to life, indicating that true brotherly love is a sign of regeneration. This love is characterized by a genuine concern for the highest interests of others within the household of faith. It is more than a verbal affirmation; it involves practical actions such as prayer, support, and kindness towards fellow believers. The presence of such love in a believer's life is both a testimony of their new birth and a delight to God, as expressed in Psalm 133:1, 'Behold, how good and pleasant it is for brethren to dwell together in unity.'