I say the truth in Christ, I lie not, my conscience also bearing me witness in the Holy Ghost, that I have great heaviness and continual sorrow in my heart. – Romans 9:1,2

There was no sham about it. It is pretty easy to work yourself up into a state of feeling, but it was not passing emotion with Paul, it was deep, true, constant grief. He says, “I say the truth in Christ, I lie not, my conscience also bearing me witness in the Holy Spirit.” He did not fancy that he felt, but he really was heart-broken for guilty souls. He did not sometimes get up into that condition or down into it, but he lived in it. “I lie not,” he says, “I do not speak more than the truth. I do not exaggerate.” For fear he should not be believed he asserts as strongly as is allowed to a Christian man— “I say the truth in Christ. I lie not.” His was true heaviness, real sorrow…Paul’s feeling was very gracious. It was not an animal feeling, or a natural feeling—it was a gracious feeling, for He says, “I say the truth in Christ.” When he was nearest to his Lord. When he felt most his union with Christ and communion with Him, then he felt that he did mourn over men’s souls.

It was truth in Christ that he was expressing because he was one with Christ! He had a love for sinners because his very soul was knit to Christ. He had a heaviness such as his Master knew when He, also, was very heavy and sweat great drops of blood in Gethsemane, in the day of His passion. O Beloved, we need the Spirit of God to work this feeling in us! It is of no use to try to get it by reading books, or to pump yourself up to it in private—this feeling is the work of God! A soul-winner is a creation. As a Christian has to be created, so out of a Christian the soul-winner has to be fashioned. There has to be a careful preparation, a softening of the soul to make the worker know how naturally to care for the welfare of others. Paul had been trained and qualified for soul-saving work. He says that his conscience bore him witness that he spoke the truth and then he says the Holy Spirit bore witness with his conscience. May we have such a manifest love for sinners that we can ask the Holy Spirit to bear witness that we have it! ~ C.H. Spurgeon

https://www.blueletterbible.org/comm/spurgeon_charles/sermons/1425.cfm