Joseph Smith gave a fascinating talk on the spirit of Elias, Elijah, and Messiah. I cannot tell whether this is the same topic as his "three grand orders of the priesthood" instruction. Yes, I know exactly that little about the whole matter. And they are even both talks about the priesthood.
Nevertheless, there are some things I would point out from what he says.
One is two distinctive uses of the word grace. In one discussion he talks about being "in grace" and compares the truth with what is found in other churches. To make sense of his conclusion requires some context.
Let us suppose a case. Suppose the great God who dwells in heaven should reveal himself to Father Cutler here, by the opening heavens, and tell him, I offer up a decree that whatsoever you seal on earth with your decree, I will seal it in heaven; you have the power then; can it be taken off? No. Then what you seal on earth, by the keys of Elijah, is sealed in heaven; and this is the power of Elijah, and this is the difference between the spirit and power of Elias and Elijah; for while the spirit of Elias is a forerunner, the power of Elijah is sufficient to make our calling and election sure; and the same doctrine, where we are exhorted to go on to perfection, not laying again the foundation of repentance from dead works, and of laying on of hands, resurrection of the dead, &c.
This spirit of Elijah was manifest in the days of the Apostles, in delivering certain ones to the buffetings of Satan, that they might be saved in the day of the Lord Jesus. They were sealed by the spirit of Elijah unto the damnation of hell until the day of the Lord, or revelation of Jesus Christ.
Here is the doctrine of election that the world has quarreled so much about; but they do not know anything about it.
The doctrine that the Presbyterians and Methodists have quarreled so much about--one in grace, always in grace, or falling away from grace, I will say a word about. They are both wrong. Truth takes a road between them both, for while the Presbyterian says: "Once in grace, you cannot fall"; the Methodist says: "You can have grace today, fall from it tomorrow, next day have grace again; and so follow on, changing continually." But the doctrine of the Scriptures and the spirit of Elijah would show them both false, and take a road between them both; for, according to the Scripture, if men have received the good word of God, and tasted of the powers of the world to come, if they shall fall away, it is impossible to renew them again, seeing they have crucified the Son of God afresh, and put Him to an open shame; so there is a possibility of falling away; you could not be renewed again, and the power of Elijah cannot seal against this sin, for this is a reserve made in the seals and power of the Priesthood.
So when comparing the word grace as used by various other religions to the truth Joseph Smith does not join with modern teachers on the matter at all. He agrees that there is a notion of being "in grace" but it is wildly different than what we are used to thinking. He says that if one falls away from it one cannot be renewed. And why not, because if one is "in grace" then one would have "tasted of the powers of the world to come". What he is saying is a continuation of what he started with, which he makes perfectly clear so he gives the doctrine of the sealing power and of making our calling and election sure, and then says "Here is the doctrine of election that the world has quarreled so much about". When Joseph Smith speaks of being "in grace" here, he is talking about one who has been sealed by up to eternal life by the power of Elijah. That is why he concludes his point by stating "the power of Elijah cannot seal against this sin, for this is a reserve made in the seals and power of the Priesthood".
Now THIS is a different notion of being "in grace" than what we are used to. Even if one disagrees that Joseph Smith was referring to having one's calling and election sure, and was to argue that he is speaking of being sealed in some other sense, one still has to concede that whatever he means it is something that one cannot be renewed if he falls away from, as that is the whole plain point of his last paragraph.
Thus one is not "in grace" if one has been baptized. For you can fall away and be renewed.
One is not "in grace" if one has the Melchizedek priesthood. For you can fall away and be renewed.
One is not "in grace" if one is sealed. For you can fall away and be renewed.
Joseph Smith teaches as plain as day here that the true doctrine of being "in grace" is something which, if one were to fall away, it would be impossible for one to be renewed again.
Now, there are other ways Joseph Smith uses the word grace. But his use of the phrase "in grace" here is specifically contrasted with the way it is used in other religions, and it means one has been sealed to eternal life by the power of Elijah. And if one falls away from it one could not be renewed because having tasted the powers of heaven, falling away would constitute crucifying the Son of God afresh.
Now there is another use of the word grace, and it is the more common usage, such as we find in Moroni and D&C 93. Grace in this sense is nothing more nor less than the overly generous and merciful gifts of God. Joseph Smith uses this meaning himself frequently, such as in this quotation:
Here, then, is eternal life--to know the only wise and true God; and you have got to learn how to be Gods yourselves, and to be kings and priests to God, the same as all Gods have done before you, namely, by going from one small degree to another, and from a small capacity to a great one; from grace to grace, from exaltation to exaltation, until you attain to the resurrection of the dead, and are able to dwell in everlasting burnings, and to sit in glory, as do those who sit enthroned in everlasting power. And I want you to know that God, in the last days, while certain individuals are proclaiming his name, is not trifling with you or me.