At what point must one be put away from the assembly as a covetous person or a railer (1 Corinthians 5:11)?
Verse 11 lists 6 people who must be removed from the assembly. Four of those are repeated from verse 10. Those four, including one who is covetous, are persons who have violated the clear statement of Gods moral law. In all four cases, one violation makes them transgressors of the law, therefore guilty. While God judges thoughts (see Matthew 5:22, 28), men can only deal with actions. When one called a brother exhibits covetousness by doing something which is, in itself, morally wrong, he must be removed from the assembly.
A railer and a drunkard complete the list in 1 Corinthians 5:11. The determination of these is more subjective, not always involving clear violations of Gods moral law. An actions circumstances or the frequency with which it occurs deserve consideration. Elders, who are spiritual, impartial, and sufficiently informed, guide the assemblys decision to put away a railer or drunkard.
Under what conditions are we not to have fellowship with one "who is called a brother" (1 Corinthians 5:11)?
Paul gave instructions to the Corinthians not to keep company with those put away, even so much as social eating (verses 9, 11). He likewise instructed the Thessalonians to withdraw from those walking disorderly (2 Thessalonians 3:6) and not to keep company with them (verse 14). When therefore the assembly must deal with an individual whose behavior endangers the behavior of others (1 Corinthians 5:6-8; 2 Thessalonians 3:11) or who needs to learn the seriousness of his sin (2 Thessalonians 3:14), social contact must be limited. In all other cases, saints should cultivate social contact with each other (Hebrews 13:1-3).
Should a person who is being disciplined by the assembly be present at the discipline meeting?
Yes. Not every form of New Testament discipline involves putting an individual away from the fellowship. How could an assembly rebuke a fellow-believer (1 Timothy 5:20) who is not present? In the case of excommunication, an individual remains part of the assembly until the assembly has acted to put him away Silently attending such a gathering might indicate submissiveness to and concurrence with the assemblys action. It may also convince a person of the sensuousness of his sin and of the saints concern for him. No one can force a person to attend; advising him to do so would be wise.
Could you explain Isaiah 7:15-16?
The context of these verses must include verse 14: "Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel. Butter and honey shall he eat, that he may know to refuse the evil, and choose the good. For before the child shall know to refuse the evil, and choose the good, the land that thou abhorrest ("fearest," JND) shall be forsaken of both her kings." Ahaz, Judahs king, feared the combined might of Syria and Israel (the 10 northern tribes). The current heir in Davids royal house thus received assurance of Judahs deliverance. The sign of this deliverance was a child named Immanuel, conceived, brought forth, and named by an unmarried woman. Although living in poverty (eating butter - or curds - and honey), the child would not be morally mature (knowing to refuse evil and choose good) before the forced removal of Israels and Syrias kings destroyed their feared alliance. Viewing this entire passage as referring to Christ (Matthew 1:22,23) brings with it the insurmountable problem of suggesting that there was a time when the Lord did not know to refuse the evil and choose the good (verses 15b, 16a). His deity demands omniscience and holiness from the moment of incarnation. In addition, the passage seems to clearly state that the promised deliverance (v.16) would come before the son (v. 14) came to moral maturity (v. 15).
Three observations may help: the word "virgin" means an unmarried woman capable of bearing children (E. Young, W. E. Vine); the passage does not sanction her naming her son "Immanuel," a name that only God could rightly bear; this is likely not the same son as foretold in 8:1. If these are reasonable, then the sign the Lord promised was fulfilled in Isaiahs day.
New Testament citations of Old Testament passages sometimes give a deeper meaning (not a differing or contradictory meaning) to that passage. In addition, we need not force every detail of a passage to conform with the prophecy or principle the Spirit highlights in a passage (see Psalm 41:9 with John 13:18; Psalm 69:4, 5 with John 15:25). Joseph, current heir of Davids royal house (Mathew 1:16), received news that a virtuous, unmarried woman (verse 20) had conceived a son. The virgin birth of the true Immanuel assured the final and spiritual deliverance of His people (verse 21); this fulfilled the deeper meaning of Isaiah 7:14.