He's a part of the "body of Christ" whose membership is known only to God.
No, that is wrong. When St. Paul wrote about people being part of the body of Christ, he referred to a specific group of Christians to whom he wrote, not to some abstract group "whose membership is known only to God".
12 Just as a body, though one, has many parts, but all its many parts form one body, so it is with Christ. 13 For we were all baptized by" one Spirit so as to form one body—whether Jews or Gentiles, slave or free—and we were all given the one Spirit to drink. 14 Even so the body is not made up of one part but of many.
15 Now if the foot should say, “Because I am not a hand, I do not belong to the body,” it would not for that reason stop being part of the body. 16 And if the ear should say, “Because I am not an eye, I do not belong to the body,” it would not for that reason stop being part of the body. 17 If the whole body were an eye, where would the sense of hearing be? If the whole body were an ear, where would the sense of smell be? 18 But in fact God has placed the parts in the body, every one of them, just as he wanted them to be. 19 If they were all one part, where would the body be? 20 As it is, there are many parts, but one body.
21 The eye cannot say to the hand, “I don’t need you!” And the head cannot say to the feet, “I don’t need you!” 22 On the contrary, those parts of the body that seem to be weaker are indispensable, 23 and the parts that we think are less honorable we treat with special honor. And the parts that are unpresentable are treated with special modesty, 24 while our presentable parts need no special treatment. But God has put the body together, giving greater honor to the parts that lacked it, 25 so that there should be no division in the body, but that its parts should have equal concern for each other. 26 If one part suffers, every part suffers with it; if one part is honored, every part rejoices with it.
27 Now you are the body of Christ, and each one of you is a part of it. 28 And God has placed in the church first of all apostles, second prophets, third teachers, then miracles, then gifts of healing, of helping, of guidance, and of different kinds of tongues. 29 Are all apostles? Are all prophets? Are all teachers? Do all work miracles? 30 Do all have gifts of healing? Do all speak in tongues? Do all interpret? 31 Now eagerly desire the greater gifts.
