To "think no evil" means to put the best possible construction on the motives and the conduct of others. It is not about evil thoughts or temptations to sin in the mind. It is about our attitude toward other people. If we are under the influence of charity we will "think no evil." Love will be our first reaction and we will not think evil until it is impossible not to. The devil has a great advantage over us in this matter because we have been hurt by the evil in others all our lives. But the Holy Spirit of God who indwells the true believer will enable him to overcome in this matter. To think evil is ungodly. To think no evil, even when the evil is obvious, is to be like Christ. We overcome evil with good, and this is a matter that is vital to our testimony as a Christian. Let this mind be in you which was also in Christ Jesus.
