2. Athil prathama rahasyam thirumanthram.
Amongst the three rahasyas the first rahasyam that a mumukshu has to know is the Thiruvashtakshara mahamanthram.
Sri PBA Swami's Sarartha Deepikai:
Amongst the three manthrams that a mumukshu has to know, the very first rahasyam he has to understand is the Thiruvashtaksharam known as the Thirumanthram. The reason that this has to be learnt first is that only after the meanings of the self is understood can one understand the meanings of the means and the goal. Therefore, the Thirumanthram which explains the nature of the self has to be learnt ahead of the other two rahasyas which explain the means and the goal. A jiva is enslaved to the Lord. He is dependent on the Lord and He enjoys nothing but his association with the Lord - this is the understanding of the self. Thirumanthram which explains this meaning is therefore to be learnt first.
The Thirumanthram is made of two parts: Pranavam and Manthrasesham. Om is the Pranavam and Namo Narayanaya is Manthrasesham. Pranavam is expanded upon by Manthrasesham. Manthrasesham is expanded upon by Dvayam. Dvayam is expanded upon by Carama Slokam. By this order also Thirumanthram has to be learnt first.
The word manthram is obtained from "manthAram thrAyathe". It is that which protects those who recite it. The protection is of two types: One is via the sound of the manthra and the second is via the meaning of the manthra.
The benefits that are obtained by those who recite it and use it in Homas is obtained via the power of the sound of Thirumanthram. The benefit accrued by the prapannas who look to nothing but the Lord, is obtained by the power of the meaning of Thirumanthram.
The second meaning is explained by Sri Bhattar in Ashtasloki as follows. If a person thinks that his body is the same as his soul then he should look at the makaram of the pranavam. Since it talks about the soul with knowledge he would understand the knowledge-less body cannot be the same as the soul. Even if one understands this, he may mistake that the soul is not dependent on the Lord and think that it is fully independent. Such a person should look at akaram in pranavam. Akaram emphasizes that the souls are enslaved to the Lord and therefore he will be cleared of his error. If he understands that a soul is enslaved to the Lord but thinks that it is similarly enslaved to many others, a fact that is commonly seen in this world, then he should look at the ukaram in pranavam. This ukaram shows that the souls are enslaved only to Sriman Narayana and as such he would correct his mistake. A person who understands all of this might still think that his own effort is sufficient to protect himself. Such a person should look at the nama: term. This term shows that the protection of the souls is the duty of the Lord and not of the self. If a person begins to develop great attachment to the people surrounding him, his wife, children, relatives, then he should look at the Narayana term. This teaches him that Sriman Narayana is the one and all relation to a soul. A person who becomes attached to things of this world should look at the "aya" term (chathurthi vibhakti). As this term signifies that eternal service to the Lord is the purpose for a soul, attachment to other things will be broken.
The thirumanthram that contains these meanings is therefore to be learnt first and is the refuge for all prapannas.