Do we not believe that a person is renewed in baptism (Rom 6:4)? Why then does one sin after baptism in spite of being renewed?
In baptism, one obtains renewal, not infallibility.
No one on earth is infallible. Notice David the prophet in the Old Testament: how the Spirit of the Lord came upon him (1 Sam 16:13) but this did not prevent him from sinning afterwards (2 Sam 24:10). Samson also, ” the Spirit of the LORD began to move upon him.” (Judg 13:25) ” And the Spirit of the LORD came mightily upon him.” (Judg 14:6), however, he sinned and broke his vow (Judg 16:19, 20).
Thus, renewal in baptism does not mean that a person does not sin thereafter.
The principle is that one’s nature becomes inclined to righteousness and sin becomes incidental.
This means that a person’s spiritual capabilities become extensive and he becomes worthy to have the Holy Spirit dwell in him through the Holy Chrism (Myron). When he sins, his conscience blames him quickly and he becomes ready to return to God.
Not to sin at all will only be realised in eternity where we shall put on the crown of righteousness. St. Paul the Apostle said, “Finally, there is laid up for me the crown of righteousness, which the Lord, the righteous Judge, will give to me on that Day, and not to me only but also to all who have loved His appearing.” (2 Tim 4:8) This means that our nature will be crowned with righteousness in the other life and will have righteousness as a nature so as not to sin afterwards. (See my book, “Life of Repentance and Purity” the Chapter on “Purity”).
Here, on earth, the righteous fall seven times and rise again (Prov 24:16).
They are still considered righteous because righteousness is the principle, whereas falling is incidental. One falls and gets purified through repentance.