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The Life That is Usable to Christ is one that He can direct and mold for His purposes. This is a life that He can direct through the working of His Spirit to accomplish His purposes. Every believer should desire to be used by God. If you have truly experienced His saving grace and realize what He has done on your behalf, then it should be a part of your inner being to desire to serve and please your God. The Love that He has shown you should be your motivation to live for Him. If God will use your life, then it must be usable to Him.
No need to question whether it would be useful, because if He made it then it is useful. But useable is another story. Useful has to be with the make-up and worth of something, but useable has to do with the objects workability and availability.
How can you be usable? To begin with, your usability is dependent upon your spiritual condition. If you are not a Believer in Christ, then you can't really be used by Him the way He wants to use you. Salvation must take place before you are useable to God.
The next requirement to being used of God is that you learn to surrender to our God. As He comes and works in your heart for the purpose of molding and shaping you it is your responsibility to be responsive to His working in your life. As He shapes and directs your job is to respond in obedience. When there is sin in a believer's life God will address it. However, He doesn't come into your problem and force you out of it, no, instead He works to alert you to the sin and then will empower you to repent of it. He will always do His part, the only breakdown is when you fail to do yours.
When you hear Him speak its your job to obey. If He points out an attitude in your heart that needs to be corrected, respond immediately. If He uncovers some activity in your life that displeases Him, repent and set up a plan in your heart to deal with it. (directly focusing on dealing with this is very important) The idea of surrender in the Christian life is simply doing what He says, when he says, How HE says. Surrender is simply allowing Him to take control and obediently following His directing.
After salvation and surrender then comes service. The one who is in tune with God and in service to Him will have no problem being usable. It will become natural to this believer to hear His voice and then respond in all things. It will become easy for God to use him. This is the place that each of us should desire to be. This is the place that is usable. If you are there, then rejoice. If you are not, then get there!
Your usability to God is dependent completely upon your willingness to get there.