“For I long to see you, that I may impart unto you some spiritual gift, to the end ye may be established; that is, that I may be comforted together with you by the mutual faith both of you and me.” (Romans 1:11-12)

In verse 15, Paul is going to state that he is ready to preach the gospel to these brethren at Rome also. He means the same thing here. The spiritual gift he will give them is spiritual knowledge attained under the preaching of the gospel.

Paul’s goal is that they be established in THE mutual faith which is the foundational set of beliefs that should be taught by all preachers.

Comfort can only be found in the doctrines of grace. Any work or will of man added to grace takes away the surety of salvation to whatever extent man must gain or maintain it. Therefore, Paul is saying he hopes to find comfort together with them by establishing their hope on the doctrines of grace.

The phrase “that is” means this is how they will be established. They will only be established if they believe the same things that Paul believes.