You failed to interpret that in light of Romans 3:12 where "there is no one who does good, not even one."
First, if you believe that Romans 3:12 and Romans 3:23 is talking about the present tense walk of believers in that they are currently sinners (in the present tense) then you must also believe that Paul is talking about how believers also currently (in the present tense) do not have any understanding and they do not seek after God, too. For it says this in Romans 3:11.
"There is none that understandeth, there is none that seeketh after God." (Romans 3:11).
Second, Scripture testifies that they used to sin as a part of their old life. For Paul says,
2 "
Wherein in time past ye walked according to the course of this world, according to the prince of the power of the air, the spirit that now worketh in the children of disobedience:
3 Among whom also we all had our conversation in times past in the lusts of our flesh, fulfilling the desires of the flesh and of the mind; and were by nature the children of wrath, even as others. " (Ephesians 2:2-3).
9 "Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind,
10 Nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the kingdom of God.
11
And such were some of you:" (1 Corinthians 6:9-11).
5 "Mortify therefore your members which are upon the earth; fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry:
6 For which things' sake the wrath of God cometh on the children of disobedience:
7
In the which ye also walked some time, when ye lived in them." (Colossians 3:5-7).