"I have not sat with the council of vanity" [Psalm 26:4]. I have not chosen to give my heart to them who endeavour to provide, what is impossible, how they may be blessed in the enjoyment of things transitory. "And I will not enter in with them that work wickedly." And since this is the very cause of all wickedness, therefore I will not have my conscience hid, with them that work wickedly.
Now he lists the things that he has accomplished through the Lord’s kindness; such kindness is the basis of his claim that he was pleasing to God. “I have not sat in the council of vanity”; he does not agree to the deliberations of the wicked by any participation in their plan. It can happen that a holy person through some chance may come to the council of the wicked where inappropriate or empty matters are put forward. But while he is aware of these things, he does not sit comfortably with them nor does he tarry over them with any delight, but either he tries to argue against the proposals or he abandons them. So while he previously denied that he sat with the wicked, now he claims that he did not enter in with the most wicked. Earlier he avoided their deliberation, later he also abandoned their actions. “Going into” pertains to a wicked action, when something begins to be done heedlessly. “Going into” indicates the beginning of the activity that the holy person professes is foreign to his conscience.
[AD 430] Augustine of Hippo on Psalms 26:4