Am I reading this right?  Is the Bible telling us that we can build up our faith by praying in the Holy Spirit?   I can tell you that is exactly what the Bible is telling us! 

You see, when a man prays in the Holy Spirit, he allows the Holy Spirit to pray "through" him, in a prayer that is perfectly in line with the Word of GOD!  No unbelief or doubt, no misconceptions of any kind.  Why?  Because the "mind" doesn't actually get involved!  THAT is why you pray in another language, or in an unknown tongue.  It is so you can pray mysteries unto GOD.  These are not mysteries to HIM, but to "YOU".  You see, if your mind could understand what you pray, it would begin to filter and ... "interrupt" (for lack of a better phrase) the prayer from the Holy Spirit and your prayer would be filtered through your beliefs, what you know and through what you hold to be true.  It would take a long time to tell you the story of how I know this to be true, but trust me when I say this: 

You should "prefer" to pray in the Holy Spirit when you have a need! 

If you want to pray in English, that is fine too!  But then, take it to the next level, and begin to pray in your most holy faith as you pray in the Holy Spirit unto GOD.  Your prayers are then exact and perfect, and totally in-line with GOD's will for you, or the situation you are praying about! 

Please take some time to experiment with this!  Don't just take my word for it!  Take some time and get alone with GOD.  Pray ... in the Holy Spirit, and just lose yourself in it! 

Be careful though!  When you come out of the prayer closet, after having spent an hour or so in this kind of prayer, you are going to see things happen around you!  You can expect anything from people looking at you strangely (because they now sense a difference in you), or even falling out in the spirit!  You may have people who all-of-a-sudden, just start confession their sins to you, or crying and asking you for prayer! 

It is the awesome and Holy presence of GOD Himself ... within you!  I really believe that if you did this long enough, you would see what people like Smith Wigglesworth or Oral Roberts saw in their lives!  Benny Hinn said when he first got born-again that he used to spend countless hours in prayer "in the Holy Spirit", and that afterwards when he would go to a meeting to speak, the Holy Spirit would just ... "show up"!  It's because when Benny walked into the room, GOD walked into the room, and HIS awesome presence came with him! 

Trust me!  Don't take my word for it ... please don't!  Try this yourself ... 
I can hardly wait to hear from you after the results!!!