One of the most famous lines the Apostle Paul wrote actually comes from the Psalms: there is none righteous, not even one (NASB; Rom 3:10; cf. Ps. 14 & 53). James agreed: We all stumble in many ways (James 3:2 NASB; cf. Prov. 20:9). Thomas Manton had some great reflections on this verse (James 3:2) and the fact that even Christians are sinful, saints and sinners at the same time:
Walk with more caution. You carry a sinning heart within you. …The man who has gunpowder with him will be afraid of sparks.
Censure others with all the more tenderness, allow for human frailty in every action (Gal. 6:1). We all need forgiveness; without grace you might fall into the same sins.
Be all the more earnest with God in asking for grace, [that] God will still keep you dependent on and indebted to his power.
Magnify the love of God with all the more praise. Paul groans under his corruptions (see the end of Rom. 7) and then admires the happiness of those who are in Christ (Rom 8:1). They had so many sins, and yet none were under condemnation.
Manton then wrote,
“Do not be altogether dismayed at the sight of failings. A godly person observed that Christians are usually to blame for three things: 1) they seek in themselves what they can only find in Christ; 2) they seek in the law what will only be found in the gospel; and 3) they seek on earth what will only be enjoyed in heaven. We complain of sin and ask, ‘when will the earthly state be free of sin?’ You should not complain but run to your Advocate.
I love those three points in this last paragraph. How true – not just back then, but even today! Indeed, we are sinful. But in Christ, by God’s sovereign grace, we are also saints, and one day we won’t have to struggle against sin any more. Until then we fight the war against sin, trusting in Christ for the victory.
The above slightly edited quotes are found in Thomas Manton’s commentary on James 3:2.