Truth is a person named Jesus Christ (John 14:6).
Truth is also the Holy Scriptures (John 17:17).
If a person want to know they abide in the truth (Christ, and His Word), they can have an assurance of this if they find they are keeping His commandments (See: 1 John 2:3). Note: This is not the ceremonial laws of the Mosaic Law like the Saturday Sabbath, circumcision, animal sacrifices, dietary laws. Instead, believers are to follow the moral laws (like do not steal, do not murder, do not covet, etc.). They are to follow the law of Christ, etcetera.
But the person who says they know the Lord and yet they do not keep His commands, they are a liar and the truth (JESUS, & His Word) is not in them.
1 John 5:12 says that he that has the Son has life, and he that does not have the Son does not have life.
So you appeal to a sinful song with affectionate terms as a part of a holier than thou diatribe.