You should earnestly seek to be whole-hearted in your Christian life and testimony. Observe how many times the phrase, "whole heart," occurs in Psalm 119. It embodies the thought that thoroughness must characterise all who have to do with God.
This was pre-eminently so in the Man, Christ Jesus. He could say, in the fullest sense of the words, "The zeal of thine house hath eaten me up." He was literally consumed with the desire to do the will of the Father Who sent Him.
So was it with the apostle Paul. Few servants of Christ, if any, had so many strings to their bow as he had. Yet he writes to the assembly in Philippi, "This one thing I do, forgetting those things which are behind, and reaching forth unto those that are before, I press toward the mark for the prize of the high calling of God in Christ Jesus!" He compares himself to a runner in the stadium, fastening his eyes upon the goal ahead where the prize is in view, and straining every nerve to be foremost in the race. He bends and concentrates his entire energies upon the attainment of the object before him. To relax his attention even for a moment would be to court failure.
So if you would gain any distinction whatever in the path of Christian faithfulness, you must be unswerving in your aim and effort. Set the Lord always before your face; fasten your eyes upon Him; firmly close your lips; and go forward, "cleaving to the Lord with purpose of heart." Even in ordinary matters of knowledge and action, the principle to which we now refer is recognized. For a man to accomplish anything of value, it is found to be absolutely essential that he should devote the whole of his powers to one end. The student commonly selects one science, which he follows exclusively. It may be geology, or botany, or any other, but no person hopes to attain eminence in more than one. Indeed, it is often found that a branch only of a single science is sufficient to occupy the studies of a lifetime. Life is so short, the ramifications of knowledge are so complicated, and man's mental abilities are so limited, that whatever the range of research may be, unless the results are collected and used for the elucidation of some particular point, time and energy are practically wasted.
Our present concern, however, is with spiritual things, and divine exhortation is independent of support from the practical wisdom of the world. I do hope you will lay it well to heart that you should make absolutely everything you engage in subserve to the interests of Christ. The one thing you are here for is to testify for the Lord. Therefore in all your reading, in all your pursuits, in your companionship and society, see that everything is made to help to a bright and faithful witness for Christ.
Remember it is the double minded who are unstable in their ways. You may have many advantages; but if you are unstable like Reuben, you surely will not excel. You cannot serve God and mammon; so the Lord Himself warned. If you seek a name and fame in the world, you will miss a place of honour in the roll of saintly worthies.
Infirmity of purpose characterised the ten revolted tribes of Israel. They were feeble-minded, and were drawn away into the heathen idolatries around. So Jehovah said of them, Ephraim hath mixed himself among the peoples; Ephraim is a cake not turned (Hosea 7:8). The figure is an expressive one. In the process of baking, the Orientals turn their cakes over and over that they may be thoroughly and evenly exposed to the influence of the heat. An unturned cake is burnt not baked, and it is of course useless, unless both sides are equally baked.
So you should remember that any attempt on your part to serve two masters must end in disastrous failure. Demas allowed his heart to set itself upon the world, and soon the difficult path of testimony grew irksome to him. The end of it was that he forsook Paul, the aged prisoner of Jesus Christ. And so will it be with you. Directly the love of the world and its things enters the soul, love for Christ and for the things above is sure to wane. It is so easy to drift into the world where there is want of watchfulness.
Bear in mind that declension always commences in the heart. Hence the exhortation, "Keep thy heart with all diligence, for out of it are the issues of life." But it is also written, "Where your treasure is, there will your heart be also." Let Christ be before your heart as your treasure. Let Him have supreme sway over the powers of your whole being. An undivided heart is the secret of true and faithful testimony for Christ. Watch therefore against anything that seeks to intrude between your heart and the person of the Lord. Upon the first sign of anything that displaces Christ in your affections, confess it at once that you may be restored. Is not the Lord worthy of the service of your whole self? Depend upon it there is no one nor anything else that is worth living for.
"There's nought on earth to rest upon;
All things are changing here,
The smiles of joy we gaze upon,
The friends we count most dear;
One Friend alone is changeless,
The One too oft forgot,
Whose love has stood for ages past;
Our Jesus changeth not.
The sweetest flower on earth,
That sheds its fragrance round,
Ere evening comes has withered,
And lies upon the ground;
This dark and dreary desert
Has only one green spot;
'Tis found in living pastures,
With Him Who changeth not."