Early in his writing to the Corinthians Paul cited that he heard "there are quarrels among you" (1Co 1:11) some saying "'I am of Paul,' and 'I of Apollos' . . . [etc.]" (:12). Later he stated "you are seeking for proof of the Christ who speaks in me" (2Co 13:3). Then as if to return the question he replied, "Test yourselves to see if you are in the faith; examine yourselves!" (:5). "Whoever confesses that Jesus is the Son of God, God abides in him, and he in God" (1Jn 4:15). When the jailer asked Paul, "'What must I do to be saved?'" (Ac 16:30) he replied, "'Believe in the Lord Jesus, and you will be saved'" (:31). When you give your testimony as a witness in court they ask you to promise that it will be "the truth, the whole truth, and nothing but the truth." Paul challenged them asking "do you not recognize this about yourselves, that Jesus Christ is in you-unless you indeed fail the test?" (2Co 13:5). What does it mean if you are "in the faith" (:5)? The preposition "in" grammatically can mean indicating a belief such as with "the one who does not work, but believes in Him who justifies the ungodly, his faith is credited as righteousness" (Ro 4:5). Therefore "God abides in him" (1Jn 4:15) because he "believes in Him" (Ro 4:5). Paul continued saying "I trust that you will realize that we ourselves do not fail the test" (2Co 13:6). This is how someone would determine if a person was a Christian or not. "We know that we abide in Him and He in us, because He has given us His Spirit" (1Jn 4:13). To abide means to stay or remain in a permanent relationship. It is not the type of question you would ask a stranger but it would be useful "in the defense and confirmation of the gospel" (Php 1:7).
Paul's question to them was if "Christ is in you?" (2Co 13:5). The preposition "in" denotes a location or place. His teaching is based on "if indeed you have heard Him and have been taught in Him, just as truth is in Jesus" (Eph 4:21). "Let that abide in you which you heard from the beginning . . . [and] you also will abide in the Son and in the Father" (1Jn 2:24). "You know Him who has been from the beginning . . . [because] the word of God abides in you" (:14). We "abide in the teaching of Christ" (2 John 9). Jesus expressed that "'My words abide in you'" (Jn 15:7) and that you "'abide in Me, and I in you'" (:4). "Just as it has taught you, you abide in Him" (1Jn 2:27). He has "given us understanding so that we may know Him who is true; and we are in Him who is true" (5:20). "After listening to the message of truth . . . having also believed, you were sealed in Him with the Holy Spirit of promise" (Eph 1:13). "The anointing which you received from Him abides in you . . . [and] His anointing teaches you about all things" (1Jn 2:27). "His seed abides in him . . . because he is born of God" (3:9). "In Him we live and move and exist" (Ac 17:28). "In Him you have been made complete" (Col 2:10). The anointing is from the "'Spirit of truth . . . [and] you know Him because He abides with you and will be in you'" (Jn 14:17). It is "for the sake of the truth which abides in us and will be with us forever" (2 John 2). "The one who abides in the teaching, he has both the Father and the Son" (2 John 9). Paul directed the Corinthians to spiritually and experientially prove themselves. They did not possess the completed New Testament as we have today but they had "no need for anyone to teach you" (1Jn 2:27).