If a preacher had one sermon to preach to unbelievers he would likely preach something that follows the Apostolic pattern in the book of Acts. But what about when faced with the chance to give just one message to those who are Christians? Here there is, naturally, a lot more liberty.
Usually, when I'm faced with a situation where I may never see the Christians I'm speaking to again this side of eternity, I speak to them about truths that are of special significance to Christians, such as the love Christ has for his bride (Eph. 3:19).
The Puritans sometimes get a bad rap for their theology, especially in the area of assurance of salvation. Yet, I gained full assurance of salvation from reading a Puritan, Thomas Goodwin. No Continental writer has quite given me a sense of Christ's love for me in the way that Goodwin did when I first read him on the heart of Christ in heaven towards sinners on earth.
So, if you ask me what topic would I speak to Christians about if I had only one study/sermon, it would probably focus on the love of Christ for the church. Indeed, I recently had the privilege of speaking on the love of Christ for his bride in Brazil when I was asked to give an impromptu bible study one evening.
How do you (a Christian) know that Christ loves you? How can you be assured of his love for you? Here below are what I believe are the three greatest reasons that Christ loves you.
1. The command of the Father on the Son. The Father gave Jesus a perpetual command to love sinners (see Jn. 6:37-40; Jn. 10:15-18; 15:10). Jesus remains in the Father's love by loving sinners. There can be no greater influence upon the Son to love us poor, miserable sinners than the command of the Father. Christ's failure to love us would actually be a failure to love his Father.
Think of Christ's words to Peter in John 21:15-17. Christ asks Peter three times, "do you love me?" Peter will show his love for Christ by feeding Christ's sheep. Now think of the Father asking the Son, "do you love me?" Son: "Yes, Father, you know that I love you." Father: "Die for my sheep, love my sheep, nourish my sheep."
Christ shows his love for the Father by loving those whom the Father has given to him. There can be no greater pleasure for Christ than expressing his love for his Father. This has massive implications for us: it means that Christ will show his love for the Father by loving us.
2. The work of the Spirit on the Son. Christ possessed the Spirit without measure (Jn. 3:34). He is the man of the Spirit, par excellence. Upon his entrance into heaven, Christ received a fresh outpouring of the Spirit to the greatest degree possible for any human (Acts 2:33; Ps. 45). As a merciful high priest, exalted in the heavens, the Spirit produces grace and mercy in Christ in a manner than even exceeded his grace and mercy on earth. Therefore, Christ, having the fruit of the Spirit (Gal. 5:22), is even more patient towards sinners in Heaven than he was on earth. This partly explains why Christ said it was better for him to go than stay (Jn. 16:7).
As Thomas Goodwin said, "your very sins move him more to pity than anger." This is what it means for Christ to be a sympathetic high priest.
Christ's resurrected body made it possible for him to receive not only a fresh outpouring of the Spirit upon his human nature in heaven, but his resurrected body enabled him to receive an even fuller outpouring of the Spirit upon his human nature in heaven, as the exalted King. Thus Christ is more patient, loving, and merciful in heaven (i.e., in glory) towards sinners on earth than when he was in his state of humiliation.
3. The holy self-love of the Son. As Christ saves and blesses his people, he is reaping the fruit of his work for sinners. He is more concerned for our salvation than we are. As a good husband, Christ loves his bride. But, remember, in loving his bride he is loving himself:
28 In the same way husbands should love their wives as their own bodies. He who loves his wife loves himself.  For no one ever hated his own flesh, but nourishes and cherishes it, just as Christ does the church,  because we are members of his body (Eph. 5:28-30).
Why would Christ deprive his own body of grace? I can be sure that he will love me because I belong to him, and he would have to hate himself before he could hate me. Whatever grace, love, blessing, etc., we have received as Christians, we can be sure that we have received these graces because Christ loves himself.
Jesus loves me, this I know, for the Bible tells me that:
1) Jesus must love sinners in order to express his love towards his Father.
2) Jesus will be patient and merciful towards me because of the effect of the Holy Spirit upon him in Heaven.
3) Jesus will love me because he is a good husband, so that by loving me more he is loving himself more.
If you're a Christian struggling with assurance, here, then, are three blessed reasons to be assured: the Father, Son, and Holy Spirit.
And guess what? How many of these reasons for Christ loving us have anything to do with things we do? The greatest reasons Christ loves you are entirely dependent not upon us, but upon the triune God, which really is good news.
The odd thing about some of the theology that comes from the camp of those who claim to emphasize grace in their preaching and teaching is that they don't always do a very good job of expressing the rich theology of grace found in the Scriptures. It is one thing to use the word grace a lot, but quite another thing to express a robust, trinitarian theology of grace that highlights the person of Christ in a manner that goes beyond over-used slogans.
Personally, I'm glad that the three greatest reasons Christ loves me are not qualifications in me, but instead dependent upon the Father, Son, and Holy Spirit.