Jesus said that there would be tares in amongst the wheat. Paul is telling those in Corinth to test to see if Christ is in them and that they are in Christ. There will always be in some churches those that are in Christ and those that are not but they look like us.
Being in the faith is being in Christ - being born of the Holy Spirit and sealed with Him for all eternity.
2 Corinthians 13:5 (NASB)
[SUP]5 [/SUP] Test yourselves to see if you are in the faith; examine yourselves! Or do you not recognize this about yourselves, that Jesus Christ is in you—unless indeed you fail the test?
We know we are in the faith because Christ lives in us. We have the witness of the Holy Spirit that we are children of God.
Fail the test - or disqualified means "rejected" - "not passing the test."
Its basic definition is “not passing the test,” “incompetent,” or “unqualified” ([FONT="Gentium" !important]Bauer[/FONT]).
Complete Biblical Library Greek-English Dictionary, The - The Complete Biblical Library Greek-English Dictionary – Alpha-Gamma.
The Father makes us "qualified" because we are in Christ..
Colossians 1:12 (NASB)
[SUP]12 [/SUP] giving thanks to the Father, who has qualified us to share in the inheritance of the saints in Light.