In salvation, the Holy Spirit unites us to Christ as our head (so we are in Christ) so that we die with him (thus he died for us, paying the penalty, the debt, for our sins as if he were us, since we are in him and so he does carry ourselves), are raised with him (thus we now live in Christ), and as he is our head, so he reigns over us, and it is no longer our own life, but his life in us, since he purchased our lives on the cross--that is what it means to say that we live in Christ. Because they are his lives, thus they ought to look like his life, and by the Spirit's uniting us to him, and by the Spirit's changing us, by showing us Christ, from one degree of glory to the next--from one degree of worship to God to the next--in that way our lives more closely resemble the lives which we in fact have as our own, though they are not our own, but Christ's, which we now live out by the power of the Holy Spirit. So, now, we have two lives: the life in Christ, the life of the Spirit, which is perfect and righteous and blameless; and the life of the flesh, the old man, which has already been killed and therefore has no power over us now, yet we return and live in that life, rather than in that life we have in Christ.
This struggle is the struggle of sanctification, yet it is not so different from salvation. Here, too, the Holy Spirit works to unite us all the more fully to Christ and therefore to live ever more fully in that life we have in Christ. Our motives will be pure when they are perfectly united to the motives of Christ, but until then we struggle with our own natures. These are not actions which we seek to do but sometimes fail, and thus say that we have failed to live in Christ, but rather it is our living in our identity in Christ, where there is no failure, since we have already been raised with Christ, and Christ's success is ours. The struggle is that our old man does not come along quietly, but we must put to death the things of the old man, and this we do by the power of the Spirit at work within us with irresistible grace to put to death the things of the old man and bring to life the things of the new man which is in Christ Jesus our Lord, now and forever, amen!
Behold, his yoke is easy and his burden light! Even what he asks, he provides the power to do, indeed he does it and on the cross it is done. All that stands for us to "do" is be changed--not that we ought to, by some power of will, change ourselves, but merely be still, let the Spirit who is in us and by whom we have been saved, if we are truly saved, change us by his power. We are transformed from those who love darkness to those who hate it and love the light, our whole outlook on life is redeemed by the grace of God, not because we have done anything, but because God has shown us grace that we might see his glory and love him and worship him with heart, mind, soul, strength; we become more really us and are better able to glorify God because we get to know him! And because we know him, because we get to see his glory--we become shaped into his likeness, and therefore become more holy, i.e. we live more rightly, and so glorify him more, which is to say that we love him more. Again: we love him, so we are changed into his image, and that is worship, which is to say adoration, of God--a circle: love, change, love, etc. And that love of God is why we are! We exist so we can love God, our getting God is our being fulfilled, we are meant, finally and ultimately, to be satisfied by and in and through God.