Christians still sin but we are not forsaken

"The adultery of David with Bathsheba is an example, not to move us to evil: but if (while we follow the way of righteousness) any chance drive us aside, that we despair not. For if we saw not such infirmities in God's elect, we which are so weak and fall so often should utterly despair and think that God had forsaken us." - William Tyndale, the Obedience of a Christian Man, pg. 163


I remember once talking to an old friend of mine from Southwestern. He was a Lordshiper who said that if we sin then it shows we were never of Him. I tried asking my friend for clarification. He kept saying the same thing every time I asked. So the conclusion is that he must have meant every word. This is the conclusion of Lordship salvation or legalism. If you sin once then you are not a Christian. However this not what Tyndale said. To be a Christian is a spiritual matter not a physical matter.

Nothing matters but that we guard the teaching and the faith

"Doctor Andreas Karlstadt has deserted us, and on top of that has become our worst enemy. May Christ grant that we be not alarmed, and give us his mind and courage, that we may not err and despair before the Satan who here pretends to vindicate the sacrament, but has much else in mind. For since he has not thus far been able to suppress with violence the whole doctrine of the gospel, he seeks to destroy it with cunning interpretation of Scripture.
Now I have foretold it, and my prophesying will become true (I'm afraid), that God will visit our ingratitude and permit the truth to be cast down, as Daniel says (Dan. 8:[12]). Because we persecute and do not accept the truth, we must again have vain error and false spirits and prophets. These have already been with us to some extent for three years, though thus far hindered by his grace. Otherwise they would long ago have wrought havoc in our ranks. Whether he will keep this disturbance in check any longer, I do not know, since no one cares, no one prays for it, and all are without fear, as though the devil were sleeping who, however, goes about as a furious lion [1 Pet. 5:8]. Although I hope restraint will not be lacking as long as I live. Therefore I personally, as long as I live, will resist insofar as God helps me, and help wherever possible. And this is my earnest, sincere warning and admonition:
First, that each one with complete earnestness pray God for a right understanding and for his holy, pure Word. In view of the fact that under such a mighty prince and god of this world - the devil -it is not within our power to preserve either the faith or God's Word, there must be divine power which protects it, as Psalm 12 well prays and says [Ps. 12:6-8], 'The promises of the Lord are promises that are pure, purified seven times. Do thou, O Lord, protect us, guard us ever from this generation. On every side the wicked prowl, as vileness is exalted among the sons of men." If we boast that we have God's word and do not take care as to how we are to keep it, it is soon lost.
Second, we, too, ought to do our part and not close our eyes, but be on our guard. For God nonetheless always holds his grace firmly over the world, so that he permits no false prophets to attempt anything except something external, such as works and subtle minute discoveries about external things. No one concerns himself with faith and a good conscience before God, but only with what glitters and shines before reason and the world. Just as the Arians apparently put up a good case in the court of reason, when they alleged that God was only one person, the Father, while the Son and the Holy Spirit were not true God.
Likewise, it was easy and pleasant for the Jews and Pelagians to believe that works without grace made one pious; and under the papacy it was said in an attractive way that the free will also contributes something toward grace. So, since it is in accord with reason, it sounds altogether pleasant to say that there is simply bread and wine in the sacrament. Who cannot believe that? If one only today would grant to the Jews that Christ was simply a man, I think it would be easy to convert them.
So our concern here should now be that we keep these two teachings far apart from each other: the one that teaches of the main articles, to govern the conscience in the spirit before God; the other, which teaches of things external or works. For more depends on the teaching of faith and a good conscience than on the teaching of good works. When works are lacking, help and counsel are at hand so that one can produce them if the teaching of faith remains firm and pure. But if the teaching of faith is placed in the background and works are put forward, then nothing can be good and there is neither counsel nor help. Then works lead to vain glory and seem to people to be something great, while God's glory disappears." - Martin Luther, Against the Heavenly Prophets, Pg. 157-158

1 Corinthians 8:11

"A much better attempt is to examine the verb apollumi and determine whether in every case it means destruction in hell. We have seen that it often does, but let us look at some of the other ninety-two instances. In Matthew 10:42 and Luke 15:4, 8, 9, 24, 32 and 33, it means 'lose': to lose a coin, to lose a son. Luke 15:17 refers to perishing of hunger. Compare Matthew 5:29, 30; 9:17; John 6:12; 2 John 8; and others.
The conclusion is that since the lost coin was later found, there is no linguistic reason to suppose that apollumi has to mean final, irretrievable destruction in hell. Therefore, only those who want to invent a contradiction in the Bible will so understand it." - Gordon H. Clark, 1 Corinthians

What Christ accomplished on the Cross was for the Elect alone

In speaking of the redemption that is in Jesus Christ, we had need to keep clear of all human systems, and hold close by the Scriptures. Jesus Christ, by the price of redemption, which he paid, delivers his people from the wrath to come, and he entitles them to an inheritance incorruptible, undefiled, and which fadeth not away: all the pains from which he delivers are eternal, and so are all the blessings he procures; every thing which takes place, by virtue of his redemption, is eternal; as he redeems not by corruptible things, when he redeems by laying down his own precious life, so nothing restored by his redemption can ever be subject to corruption. - Robert Sandeman, Letters on Theron and Aspasio

Perseverance of the Saints and its meaning

"Of course, the perseverance of the saints does not mean sinless perfection or a life free from struggle and temptation. Eradication of our corrupt nature is a long and difficult process and will not be completed until we are glorified. As long as the present life continues, we may become careless of the means of grace, our hearts may be temporarily hardened, we may fall into grievous sins. Thus we may harm others and bring temporal punishment upon ourselves. God does not promise to carry us to the skies on flowery beds of ease. But praise his name, he promises to carry, drag, or push us there. So, and only so, we arrive." - Gordon H. Clark, What Do Presbyterians Believe?