We are reading 1 Peter 1:13 and following today. 

¹³ Therefore gird up the loins of your mind, be sober, and rest your hope fully upon the grace that is to be brought to you at the revelation of Jesus Christ; ¹⁴ as obedient children, not conforming yourselves to the former lusts, as in your ignorance; ¹⁵ but as He who called you is holy, you also be holy in all your conduct, ¹⁶ because it is written, “Be holy, for I am holy.” source

Going back to verse 12 we can know the reason for the, therefore.  Verse 12 reminds us Holy Spirit has come and because He is here with us we can do this.
We can gird up the loins of our minds.
We can be sober and we can REST our hope in Jesus Christ who has been revealed to us and will yet be further revealed to us. We can be holy in our conduct. 
We do want that, don’t we?

Gird. I like the definition I found for this word: gird.

To prepare (oneself) for action

It is with our minds yielded to Holy Spirit that we are prepared for action. 
And there are plenty of actions to be taken in this present season. Right?

A silly observation and I’ll end this for today.
Loins. 
Swap the o and i  in loins and we get LIONS.
If we gird up the loins of our minds, will we become more like the LION of Judah?