The fact that God has given you faith, which when you exercise it will sustain you through adversity, is a proof that God is going to reward you and help you at the end of life’s journey. Your faith in adversity is a visible token, a manifest token, a visible proof, that your faith is operating. If you have faith when all is fine, that is nice. If it disappears when all is not fine, there is no proof that you are one of those that will be counted worthy. You will never be intrinsically worthy; it is a proof that you are counted worthy, a proof that by grace God will see you as being worthy. Our patient suffering and faith does not deserve eternal blessing, but it proves that we are one of those who by grace is under eternal blessing.
The verse begins, by the way, with the word ‘which is’ in italics, they are not in the Greek. Verses four and five read straight on in the Greek. In our day we would put a long hyphen between the word ‘endure’ and ‘a manifest’. ‘So that we ourselves glory in you in the churches of God for your patience and faith in all your persecutions and tribulations that ye endure – a manifest token of the righteous judgment of God.’ The equivalent to that long hyphen is the ‘which is’ which our translators have supplied so that it helps us.