There are four Passages in the New Testament that are unknown to most Christians:

1) Christ declared that all that are in the Graves shall hear his voice and come forth.

2) Christ went and preached to spirits that are in prison.

3) Believers were baptized for the Dead.

4) After the Resurrection of Christ, many Saints who were dead came out of their graves and appeared to many in Jerusalem.

John 5:
24 Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.
25 Verily, verily, I say unto you, The hour is coming, and now is, when the dead shall hear the voice of the Son of God: and they that hear shall live.
26 For as the Father hath life in himself; so hath he given to the Son to have life in himself;
27 And hath given him authority to execute judgment also, because he is the Son of man.
28 Marvel not at this: for the hour is coming, in the which all that are in the graves shall hear his voice,
29 And shall come forth; they that have done good, unto the resurrection of life; and they that have done evil, unto the resurrection of damnation.

1 Peter 3:
18 For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit:
19 By which also he went and preached unto the spirits in prison;
20 Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.

1 Corinthians 15:
23 But every man in his own order: Christ the firstfruits; afterward they that are Christ's at his coming.
24 Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power.
25 For he must reign, till he hath put all enemies under his feet.
26 The last enemy that shall be destroyed is death.
27 For he hath put all things under his feet. But when he saith all things are put under him, it is manifest that he is excepted, which did put all things under him.
29 Else what shall they do which are baptized for the dead, if the dead rise not at all? why are they then baptized for the dead?

Matthew 27:
51 And, behold, the veil of the temple was rent in twain from the top to the bottom; and the earth did quake, and the rocks rent;
52 And the graves were opened; and many bodies of the saints which slept arose,
53 And came out of the graves after his resurrection, and went into the holy city, and appeared unto many.