How did Luther and Calvin view Christian perfectionism? The question is as simple as the title.  Some Christians believe we can enter a state without sin.  How did Luther and Calvin view that kind of concept?
Martin Luther and John Calvin followed the tradition of St. Augustine in abhorring any theoretical belief in a state of sinlessness, whether for a moment, day, year, or whatever.

They seem to have regarded sinless perfection as the vain imagination of human pride and a result of our sinfulness.  For example, commenting on Psalms  106:6, Calvin said:


  How very abominable, then, is the pride of those who hardly imagine that they offend in the least possible way; nay, who even, like certain fanatics of the day, conceive that they have attained to a state of sinless perfection! (John Calvin, Commentary on Psalms 106:6)


The very words ‘sinless state of perfection’ even for a moment amount to near blasphemy in the mind of a person who is aware of himself and has a due regard to the nature of sin and its seriousness, according to the reformers.

Luther explains what it means to live by the Spirit not ‘sinning’ in this way:


  This life, then, is a life of being healed from sin, it is not a life of sinlessness, with the cure completed and perfect health attained. The church is the inn and the infirmary for those who are sick and in need of being made well. But heaven is the palace of the healthy and the righteous. As blessed Peter says in his Second Epistle 3:13 that the Lord will build “new heavens and a new earth in which righteousness dwells.” Righteousness does not yet dwell here, but it is preparing a dwelling place for itself here in the meantime by healing sin. All the saints have had this understanding of sin, as David prophesied in Ps. 32:5 ff. And thus they all have confessed that they were sinners. (Luther’s Works 25.262-264)


In describing what this theoretical state would actually be Luther provides a very reasonable explanation to show this can never happen for a moment on this side of heaven:


  If we were pure of all sin, and if we burned with a perfect love toward God and our neighbor, then we would certainly be righteous and holy through love, and there would be nothing more that God could require of us. That does not happen in this present life but must be postponed until the life to come. We do indeed receive the gift and the first fruits of the Spirit here (Rom. 8:23), so that we do begin to love; but this is very feeble. If we loved God truly and perfectly, as the Law requires when it says (Deut. 6:5): “You shall love the Lord your God with all your heart, etc.,” then poverty would be as pleasant for us as riches, sorrow the same as pleasure, death the same as life. Indeed, one who loved God truly and perfectly would not be able to live very long but would soon be devoured by his love. But human nature now is so submerged in sin that it cannot think or feel anything correct about God. It does not love God; it hates Him violently. Therefore, as John says (1 John 4:10), “not that we loved God but that He loved us and sent His Son to be the expiation for our sins.” (Luther’s Works Volume 27.65)


Luther quotes St. Augustine in another place to show how uniform his view is to that wise old Christian, in times past:


  Augustine says in his 29th Epistle, to blessed Jerome: “Love is the power by which a person loves what he ought to love. In some people this is stronger and in others weaker, and in still others there is none at all; but it is never in its fullest degree, so increased, in any man as long as he lives. But as long as it can be increased, what is less than it ought to be comes from sin. Because of this defect ‘there is not a righteous man on earth who does good and never sins’ (cf. 1 Kings 8:46). And because of this defect ‘no man living is righteous before God’ (Ps. 143:2). Because of this defect, ‘if we say we have no sin, we deceive ourselves, and the truth is not in us’ (1 John 1:8). Also because of this defect, no matter how much we progress, we are forced to say: ‘Forgive us our debts’ (Matt. 6:12), even though in Baptism all our sins of word, deed, and thought have been forgiven.”  So far Augustine. But the same relation as for Baptism, indeed, a much stronger one, is in force for penitence and indulgences. From all of this it is obvious that there is no sin which is venial according to its substance and its nature, but also no merit. For even the good works which are done while the tinder of sin and sensuality are fighting against them are not of such intensity and purity as the Law requires, since they are not done with all of our strength, but only with the spiritual powers which struggle against the powers of the flesh. Thus we sin even when we do good. (Luther’s Works Volume 25.278)