“I know when we sin against a brother or sister, we are also sinning against God. When we commit a sin that truly is only against God (drunkenness, cursing when no one else is around, pornography, etc.) are we also sinning against the church/body/kingdom/our brothers and sisters? Basically, are all sins against God as well as our church family?”
Jesus spoke about sinning against each other. He addressed times when “your brother has something against you” (Matt. 5:23), and He addressed situations when “your brother sins against you” (Matt. 18:15). So, He speaks from both (1) the position of the offender (i.e., the one who has committed the sin against a brother in Matthew 5:23-24) and (2) the position of the offended (i.e., the one who has been sinned against by a brother in Matthew 18:15-18).
As Jesus focuses on this issue and as He gives attention to both sides of the offense, there is indication that there is knowledge of the offense by the one on the receiving end. When I have sinned against a brother, Jesus states that my brother “has something against” me (Matt. 5:23). To have something “against” me requires knowing something that I have done. The same is true from the other side—when my brother has sinned against me, I have a knowledge of it, so that I can (and should) “go and tell him his fault” (Matt. 18:15). Neither of these positions could be addressed without there being knowledge of an offense.
This is also seen when Jesus speaks of this matter on two other occasions. Following His instructions on how to handle the matter when “your brother sins against you” (Matt. 18:15-18), Peter asked Jesus, “Lord, how often shall my brother sin against me, and I forgive him? Up to seven times?” (Matt. 18:21). It begs the question, “How would Peter know if his brother had sinned against him?” Jesus goes on to tell the Parable of the Unforgiving Servant (Matt. 18:23-35). In the parable, the unforgiving servant was aware that he was indebted to his master AND that a fellow servant was indebted to him. The debts, in this parable, represent sin, and there is an awareness of such sin against another. This is later emphasized by Jesus again when He taught, “Take heed to yourselves. If your brother sins against you, rebuke him; and if he repents, forgive him. And if he sins against you seven times in a day, and seven times in a day returns to you, saying, ‘I repent,’ you shall forgive him” (Luke 17:3-4). In order to “rebuke” a brother who sins against me, I must know of that sin having been committed against me. So, without that awareness, there is no relational breach to address through repentance, rebuke or forgiveness, as Jesus commands in these passages.
Still, we must remember that “your sin will find you out” (Num. 32:23). In other words, sin has a way of moving from the private arena (i.e., no one else knows about this) to the public arena (i.e., when someone else knows about it). In the examples used, if one uses foul language in private, it is often not long before he uses that language around “worldly friends,” who also use and accept that kind of language. Or, if one views pornography, it is often not unusual for someone (even of the world) to come across their history and for their addiction to become known. In examples like this, one’s influence for Christ (and by extension, His church) has been tarnished, and his/her once-private sin (now known by even one other person) becomes a harmful reflection on Christ and His church—bringing reproach on the church, which violates His commands of us (Matt. 5:13-16; 1 Pet. 2:11-12).
However, until a private sin becomes a known sin, there is no Scriptural grounds for stating that such sin is against our church family. That does not mean that our private sin is any less sinful or less harmful simply because it is not a known sin. All sin separates us from God (Isa. 59:1-2), and private sin can affect how we interact with our brethren in public. “None of us lives to himself” (Rom. 14:7). We are to live as “members of one another” (Rom. 12:4-5; Eph. 4:25), emphasizing our mutual responsibility to help each other reach heaven (1 Thess. 2:19-20) by keeping sin out of our lives (both privately and publicly).