Question:
What should you do if friends you had that proclaimed to be Christians, backslid and began doing illegal activities? Should you tell the authorities about those activities?

Answer:
It is important for us to realize that it is God who has given us the authorities, and we should thank Him for them, for they are given to us for our protection and peace. Without these authorities, anarchy and destruction would prevail, and we can especially thank the Lord for the laws that are in our country which are according to Scriptural principles.

But in any case, we are told: “Let every soul be subject unto the higher powers. For there is no power but of God: the powers that be are ordained of God.” In speaking of the president, policeman, school teacher, etc., we are told: “He is the minister of God to thee for good. But if thou do that which is evil, be afraid; for he beareth not the sword in vain: for he is a minister of God, a revenger to execute wrath upon him that doeth evil” (Romans 13:1,4).
Before telling the authorities, however, of the illegal activities of our friends, it may be good to go privately to the one who has sinned, and “tell him his fault between thee and him alone: if he shall hear thee, thou hast gained thy brother” (Matthew 18:15). And  if he will not hear you, maybe take one or two more with you, “that in the mouth of two or three witnesses every word may be established” (Matthew 18:16). If the person does not hear the two or three and if he is in the assembly, the Lord instructs us to tell the assembly (Matthew 18:17). We may not be free to go to our “friend,” but we should prayerfully consider what the Lord says in these verses and only go to him/her if we are doing it because we know the Lord has told us to do it.
If the above fails, we must consider telling the authorities if we know of illegal activities, because the knowledge itself (that we have), places responsibility on us to be separate from it, or we will be partakers of their evil deeds (Revelation 18:4). The Word of God is very clear about this. “And if a soul sin, and hear the voice of swearing, and is a witness, whether he hath seen or known of it; if he do not utter it, then he shall bear his iniquity” (Leviticus 5:1). We may be hesitant to tell because of fear of being called a “tattle-tale,” but many a person has been taken to jail with the guilty party, just because they knew the wrong and did not take a stand against it by telling the authorities. If we are going to represent Christ, we must stand for what is the truth. Association with evil does defile us (1 Corinthians 15:33).
We may ask: “What about the verse that says: ‘Love shall cover a multitude of sins’? (1 Peter 4:8).” This is speaking primarily about our relationship with believers, because the first part of that verse says: “And above all things have fervent love among yourselves.” We should not broadcast any sin to whoever will hear us, whether it is committed by saved or unsaved. Sin is very serious, for it caused Christ to suffer and die. Spreading it around is not love and can only make it harder to restore the erring one. But regarding the question of friends who are “doing illegal activities,” let us be assured that we are doing wrong to not try to help them stop in their evil ways, and when they break a law of the land, the authorities are given by God for this purpose.