[Along the way] they strengthened the hearts of the disciples, urging them to continue [to believe and practice] the faith and [explaining] that entering God's [heavenly] kingdom [See II Tim. 4:18] requires enduring many trials.
But if we experience trouble, it is in order [to provide] for your comfort and salvation. Or, if we are comforted, it is in order [to provide] for your comfort [also], which enables you to endure patiently the same sufferings we experience.
Now I rejoice in [spite of] my sufferings for your sake, and I am filling up [i.e., completing] in my physical body what was lacking in the afflictions Christ endured for His body’s sake, the church.