“Some Protestants suggest that Jesus’s words in Matthew 6:17 are an unconditional prohibition of the use of ashes in association with fasting (and presumably that their use at the beginning of Lent is therefore unwarranted):
But you, when you fast, anoint your head and wash your face. (Matthew 6:17)
For them it seems pretty clear that any use of ashes in association with fasting contradicts what Jesus says here and therefore constitutes disobedience to Him. This conclusion is unwarranted.
The quotation is taken from the Sermon on the Mount. Elsewhere in the same sermon the Lord Jesus says this:
So let your light shine before men, that they may see your good works, and glorify your Father who is in heaven. [Matthew 5:16]
Jesus says that one of the proper effects of our good works is to serve as a witness to others, so that they will come to glorify God as we do ourselves. This being the case it is likewise clear that to hide one’s good works at all times and in every case amounts to a direct contradiction of what He says here. We may reasonably conclude that our good works are good not just for our own souls but also for the souls of others.
The next thought to consider is whether fasting qualifies as a “good work.” I believe that this goes without saying. It is unquestionably a good work when done for the right reason: namely, as a sign of our penitence before God. I ask, then, whether there is any reason to suppose that fasting is a good work that we should let other men see? In light of Matthew 5:16 is it reasonable for others to see our penitence? Yes. There is good reason to suppose that fasting should at least sometimes be seen by others. Why? Because it is a sign of penitence, and it is absurd to suppose that men would always and only be harmed by seeing our penitence. Indeed, the fact of our repentance could very reasonably be understood by others as a reason that they too should be sorry before God for their own sins.
So fasting is a good work, and it is perfectly reasonable to hold that others may benefit from seeing us fast, and thereby come to glorify our Father who is in heaven (as Matthew 5:16 says). But fasting is something that isn’t immediately obvious. We can’t look at a man and thereby know that he is fasting. Hence the value of the sign of ashes, which are a visible sign of the inward realities of penitence and fasting. Contrary to being an evil thing, an external sign of penitence is a good thing precisely because it shows to other men that we are penitent—something that is a good work, and which therefore (in keeping with the Lord’s command in Matthew 5:16) we ought (at least sometimes) to let men see so that they too may glorify God with us.
What shall we say, then, about Matthew 6:17? Does this view of penitence as something that should at least occasionally be seen contradict what the Lord says there? No it does not. To see this we need only look at its context:
“Take heed that you do not your justice before men, to be seen by them: otherwise you shall not have a reward of your Father who is in heaven. Therefore when you do an alms-deed, sound not a trumpet before you, as the hypocrites do in the synagogues and in the streets, that they may be honoured by men. Amen I say to you, they have received their reward. But when you do alms, let not your left hand know what your right hand does. That your alms may be in secret, and your Father who sees in secret will repay you. And when you pray, you shall not be as the hypocrites, that love to stand and pray in the synagogues and corners of the streets, that they may be seen by men: Amen I say to you, they have received their reward. But you when you shall pray, enter into your chamber, and having shut the door, pray to your Father in secret, and your father who sees in secret will repay you. … And when you fast, be not as the hypocrites, sad. For they disfigure their faces, that they may appear unto men to fast. Amen I say to you, they have received their reward. But you, when you fast, anoint your head and wash your face; that you appear not to men to fast, but to your Father who is in secret: and your Father who sees in secret, will repay you.” –[Matthew 6:1-6, 16-18; emphasis added]
In context the Lord’s point is clear: when we do good, and when we give alms, and when we pray, and when we fast, our goal must not be to gain the approval of men, and we must not be hypocritical: that is, our good deeds, alms, prayers, and fasting must be genuine. In this light there is no conflict at all between the Lord’s prior command (in Matthew 5:16) to let men see our good works and these commands. We do good not for the sake of the praise of others and not as hypocrites but out of love for God, and in the hope that if men do see them, they will be moved to glorify God with us. So the point with regard to fasting (in Matthew 6:17) is not that ashes are simply out of bounds, but that we must be truly penitent.
The alternatives are ridiculous. It is absurd to think that public prayer is always hypocritical. It is absurd to think that hypocrisy is always present if a man makes known his penitence by means of ashes. Furthermore the Lord at least tacitly commends the use of ashes as a sign of penitence when He said this:
Woe to you, Corozain, woe to you, Bethsaida: for if in Tyre and Sidon had been wrought the miracles that have been wrought in you, they had long ago done penance in sackcloth and ashes. [Matthew 11:21]
Jesus says here that sackcloth and ashes are signs of the genuine repentance that would have been found in Tyre and Sidon. Consequently it is clear that He considered the use of ashes as a sign of genuine penitence to be a good thing and not evil. So the use of ashes by Catholics on Ash Wednesday is not a violation of what the Lord says in Matthew 6:17 unless a particular Catholic or other is hypocritical in receiving them. If he is not genuinely penitent, or if he receives the ashes merely for the sake of being seen to receive them, then he would indeed be violating what the Lord has said.”