Recently, someone on the internet said that once a man is justified, he becomes a saint and can no longer sin willfully. This is a Reformed doctrine which comes from a misunderstanding of St. Paul's Teaching that salvation is "not by works." But that is all it is, a misunderstanding.
Let's look at an example from Scripture.
John 15:2 Every branch in me that beareth not fruit he taketh away:
These branches that bear no fruit, are they sinners? or saints?
What does it mean that the Father takes these branches away?
These branches are saints. They have been united to Christ and are part of His Body or in this case, vine. But they have done no good deeds. Thus, they have produced no good fruits.
Notice that these could also be described as saints, since they are attached to the Vine. Yet, they are cut away and thrown into the fire. Jesus goes on to say:
6 If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them, and cast them into the fire, and they are burned.
Here's what Jesus says elsewhere of Saints who have produced no good fruit, but indeed, begun to commit sin:
Luke 12:42 And the Lord said, Who then is that faithful and wise steward, whom his lord shall make ruler over his household, to give them their portion of meat in due season?
43 Blessed is that servant, whom his lord when he cometh shall find so doing.
44 Of a truth I say unto you, that he will make him ruler over all that he hath.
45 But and if that servant say in his heart, My lord delayeth his coming; and shall begin to beat the menservants and maidens, and to eat and drink, and to be drunken;
46 The lord of that servant will come in a day when he looketh not for him, and at an hour when he is not aware, and will cut him in sunder, and will appoint him his portion with the unbelievers.
47 And that servant, which knew his lord's will, and prepared not himself, neither did according to his will, shall be beaten with many stripes.
48 But he that knew not, and did commit things worthy of stripes, shall be beaten with few stripes. For unto whomsoever much is given, of him shall be much required: and to whom men have committed much, of him they will ask the more.
Note that this individual is described as a "servant". And the "servant" calls Jesus, Lord. Yet, the servant begins to sin against his neighbors and becomes a servant of lust (unrighteousness). Thus, the servant, who is basically a saint, has turned to sin and become a willful sinner.
So, from my perspective, it is untrue that a saint can not commit willful sin. In fact, as Jesus says, the Saint who becomes a sinner, will be punished more vehemently than the sinner who did not know that he must repent of his sins to be saved.