5 This is an obvious sign of the righteous judgment of God, to the end that you may be counted worthy of God’s Kingdom, for which you also suffer. (WEB)
The sign of His righteous judgment [is found in verses 7, 9] will be the second coming of our Lord and Savior Jesus Christ. He will come down with His heavenly army to punish those who have rejected His Word and for persecuting, killing and abusing His followers.
Our suffering in this present age is to display our commitment, love, and dedication to the Lord Jesus Christ and the Scriptures. No matter what the world under the influence of the devil brings against us, through the empowerment of the Holy Spirit, we will not be moved from following Christ.
Those who suffer and do not renounce their faith in Jesus Christ are counted worthy and will be rewarded at His second coming.