George Mueller on Happiness in God

According to my judgement the most important point to be attended to is this: above all things see to it that your souls are happy in the Lord. Other things may press upon you, the Lord’s work may even have urgent claims upon your attention, but I deliberately repeat, it is of supreme and paramount importance that you should seek above all things to have your souls truly happy in God Himself! Day by day seek to make this the most important business of your life. This has been my firm and settled condition for the last five and thirty years. For the first four years after my conversion I knew not its vast importance, but now after much experience I specially commend this point to the notice of my younger brethren and sisters in Christ: the secret of all true effectual service is joy in God, having experimental acquaintance and fellowship with God Himself. 

The point is this: I saw more clearly than ever, that the first great and primary business to which I ought to attend every day was, to have my soul happy in the Lord. The first thing to be concerned about was not, how much I might serve the Lord, how I might glorify the Lord; but how I might get my soul into a happy state, and how my inner man might be nourished. For I might seek to set the truth before the unconverted, I might seek to benefit believers, I might seek to relieve the distressed, I might in other ways seek to behave myself as it becomes a child of God in this world; and yet, not being happy in the Lord, and not being nourished and strengthened in my inner man day by day, all this might not be attended to in a right spirit.

George Mueller, Biographical Sketch and Desiring God, p.357.