If you fellowship with sin, you partake of the sin (Prov. 11:27).
You tend to assume the pattern of sins and weaknesses of those with whom you associate (Prov. 13:20).
Two categories of association trespass.
The responding trespass, in which the believer admires those who are sinning and, by association with them, is influenced toward sinfulness. This is why we are warned against associating with certain types of believers (Rom. 16:17–18; 1 Cor. 5:9–11; 15:33) as well as with unbelievers (Prov. 1:10–19; 2 Cor. 6:14–16).
The reaction trespass, in which the believer reacts to a mental attitude sin. For example: Someone is implacable, so you become implacable. Someone is jealous or vindictive, so you seek to retaliate. Someone appears to be hostile to you, so you, in turn, show hostility (Prov. 24:29; Heb. 10:30).
Under reaction-type trespasses, the believer lowers himself to the standard of the offender (Rom. 12:17–19).
By applying Romans 12:19–20, the believer places the matter in the hands of the Lord and so avoids the association trespass (Heb. 1:9).
Since interaction in the human race is inevitable, the believer should guard against association sins which would rob him of his inner peace and happiness (Rom. 12:17; 16:17).