What does it mean to "put on" Christ?

Though Protestants have often tried to argue that the 'clothing analogies' in Scripture correspond to the notion of "Imputation" (having our unrighteousness covered by the imputed righteousness of Christ), upon careful examination they actually far better align with the Catholic view of salvation.

The first thing to understand when talking about the Catholic view of salvation is that there is a distinction between Nature and Grace. Protestants generally either deny this or don't have the philosophical foundation to really know what it is, but in Catholic tradition the distinction is as plain as day.

The ream of Nature is that which pertains to man as a creature, with his human abilities corresponding to the way God designed human nature to operate. Man can think, breathe, eat, see, imagine, love, etc, all according to his natural powers which his human nature makes possible for himself. Man's nature does not enable him to fly, run at high speeds, see in the dark, etc. Though these are possible within the created realm (e.g. some animals can fly, run fast, see in the dark), they don't belong to man as a natural human ability. More on this in a bit.

The realm of Grace is that which pertains to the uncreated order of existence, namely the Divine. These are things which, by definition, rise above and beyond the natural realm, into the super-natural realm. (The term "super" literally means "above and beyond".) Since God is a super-natural being, and man is a created, finite being, there is obviously an infinite gap between Creator and creature. Grace is what 'bridges' that gap (to the extent it is possible) by equipping man with super-natural gifts that enable man to now interact on a super-natural level with God.

While man can naturally love, naturally believe, naturally hope, etc, these natural actions don't transcend the created order. As an example, a pagan can believe in God and love God, but this is a mere natural-level human faith and human love. These are not sinful actions, but rather they, by definition, fall short of the super-natural Intimacy that God desires. God desires the intimacy with us on a deep level like the intimacy husband and wife share, but without grace the intimacy level will fall very short of deep intimacy, such as that of loving food and 'having faith' that a bridge wont collapse when you're on it.

Back to the comment that man cannot fly, run at high speeds, and see in the dark, etc, there are tools that can enable man to do this. An airplane enables man to fly, a car enables man to run at high speeds, a flashlight lets man see in the dark. These natural 'accessories' change how man lives. Similarly, grace is a super-natural 'accessory' that enables man to change the way he lives. Grace equips man with the super-natural level of believing and loving, rising (literally) infinitely above the natural level of believing and loving. And as with the human examples of plane, car, and flashlight, preserving man's natural integrity, Grace also isn't destroying or removing human nature, but rather 'elevating' it's performance to new, unimaginable heights.

Now when examining the 'clothing analogies' in Scripture, keep the Nature and Grace distinction in mind, for it's the lens by which these analogies make the most sense. 

The Greek word endyo, which simply means "to clothe" (or "put on"), is used in both a literal and metaphorical way throughout Scripture. Paul frequently uses the term in his Epistles, so I will look at the analogies he employs when using the word: 

Rom 13:12 So then let us cast off the works of darkness and put on the armor of light. ... 14 But put on the Lord Jesus Christ, and make no provision for the flesh, to gratify its desires.

Gal 3:27 For as many of you as were baptized into Christ have put on Christ.

Eph 4: 22 to put off your old self, which belongs to your former manner of life and is corrupt through deceitful desires, 23 and to be renewed in the spirit of your minds, 24 and to put on the new self, created after the likeness of God in true righteousness and holiness.

Eph 6: 11 Put on the whole armor of God, that you may be able to stand against the schemes of the devil. ... 14 Stand therefore, having fastened on the belt of truth, and having put on the breastplate of righteousness

Col 3: 9 Do not lie to one another, seeing that you have put off the old self with its practices 10 and have put on the new self, which is being renewed in knowledge after the image of its creator. 11 Here there is not Greek and Jew, circumcised and uncircumcised, barbarian, Scythian, slave, free; but Christ is all, and in all. 12 Put on then, as God's chosen ones, holy and beloved, compassionate hearts, kindness, humility, meekness, and patience... 14 And above all these put on love, which binds everything together in perfect harmony.

1 Thess 5:8 But let us, who are of the day, be sober, putting on the breastplate of faith and love; and for an helmet, the hope of salvation.

While it might be tempting to read texts that speak of "putting on Christ" as having "Christ's Righteousness Imputed," this interpretation immediately falls flat when the texts in question are consulted. For example, putting on armor suggests equipping someone, building on their nature; not imputing armor to them even though they're actually unarmed. Same thing with putting on love, kindness, humility, meekness, etc. These are inward dispositions, not something external that is imputed. The context in which these virtues are stated clearly indicate they are special to the Christian alone, which corresponds to grace building on nature. And the notion of putting on "the new self" also doesn't fit with an external imputation, since the point is that the Christian is a new creation, a radical inward change has happened. This is why each time "put on the new self" is mentioned, the context is that of "renewal of minds" and "created after the likeness of God".

Thus, the "putting on Christ" most certainly corresponds to having Christ indwell in your heart (Jn 14:23), as Paul says: "that Christ may dwell in your hearts through faith" (Eph 3:17). And Galatians 3:27 is especially clutch in this regard, for it says that it is the Baptized who have "put on Christ," which is a huge contradiction to "putting on Christ" by faith alone. In Paul's mind, Baptism is understood to be transformative (e.g. Col 2:12; Rom 6:6).

The Protestant is really trapped on various fronts here. And if they say "putting on Christ" isn't a matter of justification, then there goes any hope of pinning it to the Imputation of Christ's Righteousness.

Two texts that show this "putting on" can only refer to the grace-building-on-nature paradigm are the following: Luke 24:49, which speaks of the Apostles being "clothed with power from on high," speaking of the Holy Spirit actually coming upon them and transforming them at Pentecost. And 1 Corinthians 15:53, where Paul says mortality must "put on immortality," referring to the fact our resurrected bodies will still be our human bodies but graced with a special grace that prevents us from experiencing pain and death. There is no "imputing" going on here.

The Bible also talks about the white robes of the saints in Heaven (Rev 6:10-11; 7:14). The description given shows that this signifies a pure soul, not some external covering. In Our Lord's parable of the wedding garment in Matthew 22:10-13, tradition has interpreted this "wedding garment" as being in a state of grace, being worthy to enter Heaven. This signifies a personal holiness, not a legal imputed holiness of another that ignores the inward heart of the wedding guest. The parable of the Prodigal Son takes on the same analogy when the father order the servants to clothe the returned son with a new expensive robe, signifying a restored status. 

Lastly, I will look at a passage that Protestants frequently point to, Zechariah 3: 

3 Now Joshua was standing before the angel, clothed with filthy garments. 4 And the angel said to those who were standing before him, “Remove the filthy garments from him.” And to him he said, “Behold, I have taken your iniquity away from you, and I will clothe you with pure vestments.” 5 And I said, “Let them put a clean turban on his head.” So they put a clean turban on his head and clothed him with garments. And the angel of the Lord was standing by. 6 And the angel of the Lord solemnly assured Joshua, 7 “Thus says the Lord of hosts: If you will walk in my ways and keep my charge, then you shall rule my house and have charge of my courts, and I will give you the right of access among those who are standing here.

In this analogy, Joshua is lacking clean garments to be able to stand before God and needs God to give him clean ones. Given that the Catholic clothing paradigm is already well established and the Protestant clothing Imputation paradigm isn't, the Catholic paradigm should be given preference here.

In this analogy, Catholics would view the pure vestments as Grace that 'builds on' Joshua's nakedness (human nature). It's not that Joshua's nakedness is an evil thing in itself, but rather it is bad because in the context of appearing in God's house one must be properly dressed. On the Day of Atonement, the High Priest had to put on special robes (not just regular clothing) to enter the Holiest place (Lev 16:23), which surely signifies that we cannot enter Heaven under normal circumstances. 

The advantage of the Catholic view with this analogy is basically this: Catholics don't need to separate Justification and Sanctification the way Protestants do. Protestants say the inner sanctification of a person is different from the outer righteousness of a person, so in this analogy even though Joshua appears outwardly righteous, inwardly he is still unsanctified. In other words, in the Protestant view Joshua is legally entitled to enter God's house because he is wearing the proper clothing, but his heart is not necessarily clean. Obviously, it's absurd to say a person is legally entitled to enter Heaven even though inwardly their heart is polluted by lusts. In the Catholic view, Justification is based upon Sanctification, so the cleaning of the filth on Joshua and the clean robe signifies that he isn't just clean outwardly, but inwardly as well.

When all is said and done, the clothing analogies really support the Catholic view far better than the Protestant view. And even if the Protestant really pushed examples like Zechariah 3 or Issac seeing Jacob dressed in the fancy clothes of Esau (Gen 27:15), at most the Protestant could hope for is a draw (which doesn't help the Reformation). More importantly though, the Protestant view certainly has no merit with Paul's clothing analogies, nor is Imputation supported anywhere in Scripture to even give it a place at the possible-interpretations-table. So the Catholic can be very confident that the clothing analogies are indeed fashionably Catholic.

UPDATE: Someone in the comment box pointed out another great point from Revelation 3:4-5,

Thou hast a few names even in Sardis which have not defiled their garments; and they shall walk with me in white: for they are worthy. He that overcometh, the same shall be clothed in white raiment; and I will not blot out his name out of the book of life, but I will confess his name before my Father, and before his angels.

He pointed out that this "defiling garments" cannot refer to man's nature, because all men are born sinners. And equally this cannot be Christ's Imputed Perfect Righteousness, since this cannot be defiled by definition. So this garment can only refer to being clothed with grace and defiling this through grave sin.