I found these passages from the "Address to Students of Divinity" at the beginning of John Brown's A Compendious View of Natural and Revealed Religion in Seven Books to be very memorable and valuable, not only for those in the ministry or hoping to be, but I think for everyone.
Instruction:—If your indwelling pride be allowed to choose your company, your dress, your victuals, nay, your text, your subject, your order, your language;—if it be allowed to indite your thoughts, and, to the reproach and blasting of the gospel of Christ , to deck your sermon with tawdry ornaments and fancies, as if it were a stage-play, to blunt and muffle up his sharp arrows with silken smoothness and swollen bombast;—it if be allowed to kindle your fervour, and form your looks, your tone, your action;—or to render you enraptured or self-conceited, because of subsequent applause;—or sad and provoked, because your labours are contemned, how dreadful is your danger and that of your hearers! How can ministerial labours, originating in pride, spurred on by the fame of learning, diligence, or holiness,—hurt the interests of Satan, from whose influence they proceed:—If pride be allowed to cause you to envy or wound the characters of such as differ from, or outshine you, or to make you reluctant to Christian reproof from your inferiors, how fearful is your guilt and danger! Pride indulged is no more consistent with a Christian character, than drunkenness and whoredom.
Encouragement:See that ye, as workmen who need not be ashamed, earnestly labour rightly to divide the word of truth, according to the capacities, necessities, and particular occasions of your hearers, giving every one of them their portion in due season. Never make your own ease, your inclination of or honour, but the need of souls, and the glory of Christ, the regulator in your choice of subjects. Labour chiefly on the principal points of religion: To bring down the fundamental mysteries of the gospel to the capacities of your hearers, and inculcate on their consciences the great points of union to and fellowship with Christ, regeneration, justification, and sanctification,—these will require all your grace, learning, and labour. Never aim at tickling the ears or pleasing the fancies of your hearers, but at convincing their consciences, enlightening their minds, attracting their affections, and renewing their wills, that they may be persuaded and enabled to embrace and improve Jesus Christ as freely offered to them in the gospel, for wisdom, righteousness, sanctification, and redemption. Labour to preach the law as a broken covenant,—the gospel of salvation,—and the law as a rule of life,—not only in their extensive matter, but also in their proper order and connection. It is only when they are properly connected, that the precious truths of God appear in their true lustre and glory. It is at your infinite hazard, and the infinite hazard of them that hear you, if you, even by negligence, either blend or put asunder that law and gospel which Jesus Christ has so delightfully joined together. No where is it more necessary to take heed, than in preaching up the duties of holiness. Let all be founded in union to and communion with Christ, all enforced by the pattern, love, righteousness, and benefits of Christ, Eph. 4,5,6; Col. 3,4; 1 Pet. 3,4.
(Some paragraph breaks introduced for readability.)Always improve and live on that blessed encouragement which is offered to you as Christians and ministers in the gospel. Let all your wants be on Christ. My God shall supply all your need, according to his riches in glory by Christ Jesus. Cast all your cares on him, for he careth for you. Cast all your burdens on him, and he will sustain you. If your holy services, through your mismanagement, occasion your uncommon guilty, his blood cleanseth from all sin. You have an Advocate with the Father, Jesus Christ the righteous, who is the propitiation for your sins.
If you be often difficulted how to act, he hath said, The meek will he guide in judgment: the meek will he teach his way.—I will instruct thee, and teach thee in the way which thou shalt go. I will guide thee with mine eye set upon thee. I will lead the blind in a way which they know not.—
If you be much discouraged because of your rough way and your want of strength, he has said, When the poor and needy seek water and there is none, and their tongue faileth for thirst, I the Lord will hear them, I the God of Israel will not forsake them. I will open rivers in high places. Fear not, for I am with thee: Yea, I will help thee: I will uphold thee with the right hand of my righteousness. Fear not, worm Jacob,—I will help thee, saith the Lord thy Redeemer. I will make thee a new sharp thrashing instrument, and thou shalt thrash the mountains. My grace shall be sufficient for thee: for my strength is made perfect in weakness. As thy days are, so shall thy strength be.—
If your troubles be many, he hath said, When thou passest through the waters, I will be with thee:—the rivers shall not overflow thee: When thou walkest through the fire, thou shalt not be burnt, nor shall the flame kindle upon thee:—
If your incomes be small and pinching, Ye know the grace of our Lord Jesus Christ, that though he was rich, yet for our sakes he became poor, that we through his poverty might be rich. He shall see his seed,—the travail of his soul, and be satisfied: and he has promised, I will abundantly bless her provision, and satisfy her poor with bread. I will satiate the soul of her priests with fatness. A salary of remarkable fellowship with Christ, and of success in winning souls, is the most delightful and enriching.—
If your labours appear to have little success, be the more diligent and dependent on Christ. Never mourn as they that have no hope. Let not the eunuch say, I am a dry tree. Jesus hath said, I will pour water on him that is thirsty, and floods on the dry ground. I will pour my Spirit on thy seed, and my blessing on thine offspring. A seed shall serve him. The whole earth shall be filled with his glory. The kingdoms of this world shall become the kingdoms of our Lord and his Christ.
Believe it on the testimony of God himself: believe it on the testimony of all his faithful servants: and, if mine were of any avail, I should add it, That there is no Master so kind as Christ; no service so pleasant and profitable as that of Christ:; and no reward so full, satisfying, and permanent as that of Christ. Let us, therefore, begin all things from Christ; carry on all things with and through Christ; and let all things aim at and end in Christ.
It bears pointing out that this is the only systematic theology text I know of that is explicitly given a covenantal structure.