What is the Gospel?The Greatest News
But evil men and impostors will proceed from bad to worse, deceiving and being deceived. You, however, continue in the things you have learned and become convinced of, knowing from whom you have learned them, and that from childhood you have known the sacred writings which are able to give you the wisdom that leads to salvation through faith which is in Christ Jesus. All Scripture is inspired by God and profitable for teaching, for reproof, for correction, for training in righteousness; so that the man of God may be adequate, equipped for every good work. NASB
13. But wicked men and impostors
This is the most bitter of all persecutions, when we see wicked men, with their sacrilegious hardihood, with their blasphemies and errors, gathering strength. Thus Paul says elsewhere, that Ishmael persecuted Isaac, not by the sword, but by mockery (Galatians 4:29). Hence also we may conclude, that in the preceding verse, it was not merely one kind of persecution that was described, but that the Apostle spoke, in general terms, of those distresses which the children of God are compelled to endure, when they contend for the glory of their Father.
I stated, a little before, in what respect they shall grow worse and worse; for he foretells not only that they will make obstinate resistance, but that they will succeed in injuring and corrupting others. One worthless person will always be more effectual in destroying, than ten faithful teachers in building, though they labor with all their might. Nor are there ever wanting the tares which Satan sows for injuring the pure corn; and even when we think that false prophets are driven away, others continually spring up in other directions.
Again, as to the power of doing injury,  it is not because falsehood, in its own nature, is stronger than truth, or that the tricks of Satan exceed the energy of the Spirit of God; but because men, being naturally inclined to vanity and errors, embrace far more readily what agrees with their natural disposition, and also because, being blinded by a righteous vengeance of God, they are led, as captive slaves, at the will of Satan.  And the chief reason, why the plague of wicked doctrines is so efficacious, is, that the ingratitude of men deserves that it should be so. It is highly necessary for godly teachers to be reminded of this, that they may be prepared for uninterrupted warfare, and may not be discouraged by delay, or yield to the haughtiness and insolence of adversaries.
14. But as for thee, continue in those things which thou hast learned
Although wickedness prevail, and push its way forward, he advises Timothy nevertheless to stand firm. And undoubtedly this is the actual trial of faith, when we offer unwearied resistance to all the contrivances of Satan, and do not alter our course for every wind that blows, but remain steadfast on the truth of God, as on a sure anchor.
Knowing from whom thou hast learned them – This is said for the purpose of commending the certainty of the doctrine; for, if any one has been wrong instructed, he ought not to persevere in it. On the contrary, we ought to unlearn all that we have learned apart from Christ, if we wish to be his disciples; as, for example, it is the commencement of our pure instruction in the faith to reject and forget all the instruction of Popery. The Apostle therefore does not enjoin Timothy to defend indiscriminately the doctrine which has been delivered to him, but only that which he knows to be truth; by which he means, that he must make a selection.  Besides, he does not claim this as a private individual, that what he has taught shall be reckoned to be a divine revelation; but he boldly asserts his own authority to Timothy, who, he was aware, knew that his fidelity and his calling had been proved. And if he was fully convinced that he had been taught by an Apostle of Christ, he concluded that therefore it was not a doctrine of man, but of Christ.
This passage teaches us, that we ought to be as careful to guard against obstinacy in matters that are uncertain, (such as all the doctrines of men are,) as to hold within unshaken firmness the truth of God. Besides, we learn from it, that faith ought to be accompanied by prudence, that it may distinguish between the word of God and the word of men, so that we may not adopt at random everything that is brought forward. Nothing is more inconsistent with the nature of faith than light credulity, which allows us to embrace everything indiscriminately, whatever it may be, and from whomsoever it proceeds; because it is the chief foundation of faith, to know that it has God for its author.
And which have been intrusted to thee –  When he adds, that the doctrine had been intrusted to Timothy, this gives (au]xhsin) additional force to the exhortation; for to “commit a thing in trust” is something more than merely to deliver it. Now Timothy had not been taught as one of the common people, but in order that he might faithfully deliver into the hands of others what he had received.
15. And that from (thy) childhood
– This was also no ordinary addition, that he had been accustomed, from his infancy, to the reading of the Scripture; for this long habit may make a man much more strongly fortified against every kind of deception. It was therefore a judicious caution observed in ancient times, that those who were intended for the ministry of the word should be instructed, from their infancy, in the solid doctrine of godliness, that, when they came to the performance of their office, they might not be untried apprentices. And it ought to be reckoned a remarkable instance of the kindness of God, if any person, from his earliest years, has thus acquired a knowledge of the Scriptures.
Which are able to make thee wise unto salvation – It is a very high commendation of the Holy Scriptures, that we must not seek anywhere else the wisdom which is sufficient for salvation; as the next verse also expresses more fully. But he states, at the same time, what we ought to seek in the Scripture; for the false prophets also make use of it as a pretext; and therefore, in order that it may be useful to us for salvation, it is necessary to understand the right use of it.
Through faith, which is in Christ Jesus – What if any one give his whole attention to curious questions? What if he adhere to the mere letter of the law, and do not seek Christ? What if he pervert the natural meaning by inventions that are foreign to it? For this reason he directs us to the faith of Christ as the design, and therefore as the sum, of the Scriptures; for on faith depends also what immediately follows.
16.All Scripture; or, the whole of Scripture;
though it makes little difference as to the meaning. He follows out that commendation which he had glanced at briefly. First, he commends the Scripture on account of its authority; and secondly, on account of the utility which springs from it. In order to uphold the authority of the Scripture, he declares that it is divinely inspired; for, if it be so, it is beyond all controversy that men ought to receive it with reverence. This is a principle which distinguishes our religion from all others, that we know that God hath spoken to us, and are fully convinced that the prophets did not speak at their own suggestion, but that, being organs of the Holy Spirit, they only uttered what they had been commissioned from heaven to declare. Whoever then wishes to profit in the Scriptures, let him first of all, lay down this as a settled point, that tine Law and the Prophets are not a doctrine delivered according to the will and pleasure of men, but dictated by the Holy Spirit.
If it be objected, “How can this be known?” I answer, both to disciples and to teachers, God is made known to be the author of it by the revelation of the same Spirit. Moses and the prophets did not utter at random what we have received from their hand, but, speaking at the suggestion of God, they boldly and fearlessly testified, what was actually true, that it was time mouth of the Lord that spake. The same Spirit, therefore, who made Moses and the prophets certain of their calling, now also testifies to our hearts, that he has employed them as his servants to instruct us. Accordingly, we need not wonder if there are many who doubt as to the Author of the Scripture; for, although the majesty of God is displayed in it, yet none but those who have been enlightened by the Holy Spirit have eyes to perceive what ought, indeed, to have been visible to all, and yet is visible to the elect alone. This is the first clause, that we owe to the Scripture the same reverence which we owe to God; because it has proceeded from him alone, and has nothing belonging to man mixed with it
And is profitable – Now follows the second part of the commendation, that the Scripture contains a perfect rule of a good and happy life. When he says this, he means that it is corrupted by sinful abuse, when this usefulness is not sought. And thus he indirectly censures those unprincipled men who fed the people with vain speculations, as with wind. For this reason we may in the present day, condemn all who, disregarding edification, agitate questions which, though they are ingenious, are also useless. Whenever ingenious trifles of that kind are brought forward, they must be warded off by this shield, that “Scripture is profitable.” Hence it follows, that it is unlawful to treat it in an unprofitable manner; for the Lord, when he gave us the Scriptures, did not intend either to gratify our curiosity, or to encourage ostentation, or to give occasion for chatting and talking, but to do us good; and, therefore, the right use of Scripture must always tend to what is profitable. 
For instruction – Here he enters into a detailed statement of the various and manifold advantages derived from the Scriptures. And, first of all, he mentions instruction, which ranks above all the rest; for it will be to no purpose that you exhort or reprove, if you have not previously instructed. But because “instruction,” taken by itself, is often of little avail, he adds reproof and correction.
It would be too long to explain what we are to learn from the Scriptures; and, in the preceding verse, he has given a brief summary of them under the word faith. The most valuable knowledge, therefore, is “faith in Christ.” Next follows instruction for regulating the life, to which are added the excitements of exhortations and reproofs. Thus he who knows how to use the Scriptures properly, is in want of nothing for salvation, or for a Holy life. Reproof and correction differ little from each other, except that the latter proceeds from the former; for the beginning of repentance is the knowledge of our sinfulness, and a conviction of the judgment of God.Instruction in righteousness means the rule of a good and holy life.
17. That the man of God may be perfect
– Perfect means here a blameless person, one in whom there is nothing defective; for he asserts absolutely, that the Scripture is sufficient for perfection. Accordingly, he who is not satisfied with Scripture desires to be wiser than is either proper or desirable.
But here an objection arises. Seeing that Paul speaks of the Scriptures, which is the name given to the Old Testament, how does he say that it makes a man thoroughly perfect? for, if it be so, what was afterwards added by the apostles may be thought superfluous. I reply, so far as relates to the substance, nothing has been added; for the writings of the apostles contain nothing else than a simple and natural explanation of the Law and the Prophets, together with a manifestation of the things expressed in them. This eulogium, therefore, is not inappropriately bestowed on the Scriptures by Paul; and, seeing that its instruction is now rendered more full and clear by the addition of the Gospel, what can be said but that we ought assuredly to hope that the usefulness, of which Paul speaks, will be much more displayed, if we are willing to make trial and receive it?