Like some books, some tools, and some furniture, the non-Christian is useless to God, as was Onesimus, Philemm 1:11 who formerly was useless to you, but now is useful both to you and to me.
In Christ, it is such a privilege, such a blessing to be useful in any way, Heb 6:7 For ground that drinks the rain which often falls on it and brings forth vegetation useful to those for whose sake it is also tilled, receives a blessing from God. Like the soldier, the ballplayer, or the son, it is such a privilege to be sendable in the kingdom of God. Do you feel it? Isaiah did, Isa 6:8 Then said I, Here am I; send me. What words of honor are spoken of Mark, 2Ti 4:11 Pick up Mark and bring him with you, for he is useful to me for service.
In the physical realm, usefulness depends on qualifications. So it is also in the spiritual. Here is a great promise, but there are conditions, 2Ti 2:21 Therefore, if anyone cleanses himself from these things, he will be a vessel for honor, sanctified, useful to the Master, prepared for every good work. Take the weeds out of your patch and you will be more useful. Improve your serve and you won’t be so much on the bench. Get up and you will find the father has plenty of work that could be done. Strengthen yourself and you will be given heavier work and greater privileges – labors that are not in vain in the Lord, 1 Cor 15.58.
– Bob Jennings