Alma 5:46 - 5:50

Alma 5:46 Behold, I say unto you they are made known unto me by the Holy Spirit of God. Behold, I have fasted and prayed many days that I might know these things of myself. And now I do know of myself that they are true; for the Lord God hath made them manifest unto me by his Holy Spirit; and this is the spirit of revelation which is in me.

Alma wanted to know all these things for himself. He did not want to rely on someone else’s testimony for his personal conviction. To arrive at this, he prayed and fasted many days to gain this knowledge. That knowledge was given to him by the Holy Ghost. This knowledge from an external source is called the spirit of revelation.
Here we have the definition of “spirit of revelation.” This term is mentioned many times in the Book of Mormon. It is only place that mentions this method of knowing the truthfulness of things. It must have been that which testified to me the truthfulness of the church even though I did not actually pray or fast for the knowledge. It was just true when I first heard about it from the missionaries. I have wondered if I was missing out by not having to go through this method to gain a testimony. Rather glad I did not, that I was ready and open to receive it when it did come.

Alma 5:47 And moreover, I say unto you that it has thus been revealed unto me, that the words which have been spoken by our fathers are true, even so according to the spirit of prophecy which is in me, which is also by the manifestation of the Spirit of God.

Not only did Alma receive a testimony of the gospel, but he also received a testimony that the words of the prophets as recorded in the scriptures are true as well. This is referred to as the spirit of prophecy which is also a function of the Holy Ghost.
If I understand the spirit of prophecy correctly, it allows a person to see the application of the scriptures to current or future events which is what Alma tells us in the next verses

Alma 5:48 I say unto you, that I know of myself that whatsoever I shall say unto you, concerning that which is to come, is true; and I say unto you, that I know that Jesus Christ shall come, yea, the Son, the Only Begotten of the Father, full of grace, and mercy, and truth. And behold, it is he that cometh to take away the sins of the world, yea, the sins of every man who steadfastly believeth on his name.

As a result of his personal testimony of the Jesus and the knowledge gain through scripture study, he can say for a certainty that Jesus will come to the earth as a human being and the only son of God born under these conditions. His mission will be to atone for the sins of all of us who follow His direction. Jesus is doing this because we cannot do it for ourselves and He because of His great love for all his brothers and sisters, He wanted to perform this act in our behave.
The term used here is “full of grace and mercy.” These are very misunderstood phrases in scripture. Their meaning have cause the create of creeds. To me it means that because I cannot atone for my sins, someone else has to do it for me. Who would that person be, someone who loved me enough to do so and who had the power to do so. I brought children into the world so I could not only have that association but that I could have them grow and develop into good people. That is grace to me. It is simply someone doing something for another that they could not do for themselves out regard for their situation and their love of them.

Alma 5:49 And now I say unto you that this is the order after which I am called, yea, to preach unto my beloved brethren, yea, and every one that dwelleth in the land; yea, to preach unto all, both old and young, both bond and free; yea, I say unto you the aged, and also the middle aged, and the rising generation; yea, to cry unto them that they must repent and be born again.

It is because of the testimony of Jesus, a knowledge of His coming in the future to perform the atonement and the love Jesus has for us which we need to also have for each other that Alma is preaching to all the people of the land without regard to age and sex. All must go through the process of repentance and start living as they should in order to be saved.

Alma 5:50 Yea, thus saith the Spirit: Repent, all ye ends of the earth, for the kingdom of heaven is soon at hand; yea, the Son of God cometh in his glory, in his might, majesty, power, and dominion. Yea, my beloved brethren, I say unto you, that the Spirit saith: Behold the glory of the King of all the earth; and also the King of heaven shall very soon shine forth among all the children of men.

The Spirit has called out to Alma to call out to the people, that Jesus will be coming soon. The kingdom of heaven will soon be setup. Jesus will come with all the power and authority from the Father and will enlighten all the people who will listen to him.