"Your brother will rise again." (John 11:23)

When Martha heard that Jesus was coming, she went out to meet him, but Mary stayed at home. "Lord," Martha said to Jesus, "if you had been here, my brother would not have died. But I know that even now God will give you whatever you ask." Jesus said to her, "Your brother will rise again." (John 11:20-22)

Why did Jesus say Lazarus 'will rise again'?

Jesus is consoling Martha, who is regretting Jesus not being there earlier. Martha is upset that her brother Lazarus died and knows that if Jesus had been there earlier he would have prevented it.

Jesus responds by assuring Martha that she has not lost her brother. And Martha also knows this from Jesus' teachings as she responds:
Martha answered, "I know he will rise again in the resurrection at the last day." (John 11:24)
"The last day" is being translated from the Greek phrase, ἐσχάτῃ ἡμέρᾳ. Yes, these two words can literally mean, "last day." But ἐσχάτῃ can mean "the end" as well, and ἡμέρᾳ can mean "a series of days" or "an age" according to Thayer's lexicon.

In other words, Martha is referring to the end of our physical lifetime. This is when the soul will rise at the time of death - at the end of our lifetime in this body.

Martha was responding like this because this is what Jesus had taught her. She was faithfully repeating Jesus' teachings.

Why did Martha think God gives Jesus whatever he asks?

Martha's statement is important because it explains the process of Jesus' miracles. "But now I know" indicates that Martha has been a student of Jesus' teachings, and understands the relationship between Jesus and God. She is clear, in other words, about Jesus' role in relation to God.

She is clear that Jesus and the Supreme Being are two independent persons. For Jesus to ask God to bring Lazarus' body back to life (bring his spirit-person back into his body) means that God is ultimately performing the miracle. 

It also means that Jesus is God's subordinate, since Jesus is asking the Supreme Being to do it. This elucidates Jesus' role as God's loving servant.

Martha has not confused the Supreme Being with Jesus, as some of today's teachers have. These teachers are claiming that Jesus is the Supreme Being. This is an abomination of Jesus' teachings and his whole life. It is completely offensive to Jesus, as Jesus stated directly:
"Not everyone who says to me, 'Lord, Lord,' will enter the kingdom of heaven, but only he who does the will of my Father who is in heaven. Many will say to me on that day, 'Lord, Lord, did we not prophesy in your name, and in your name drive out demons and perform many miracles?' Then I will tell them plainly, 'I never knew you. Away from me, you evildoers!'" (Matt. 7:21-23)
This statement confirms Martha's statement about Jesus' role and position in relation to God. Jesus is clearly stating that it isn't enough to be extolling and praising Jesus as "Lord, Lord." Even if someone is performing miracles and prophecy in the name of Jesus, Jesus does not accept them. He even says, "I never knew you." and "Away from me, you evildoers!" 

This illustrates clearly that Jesus is offended by those who worship Jesus as though he were God - essentially ignoring the Supreme Being.

Rather, Jesus says clearly that it is "only he who does the will of my Father who is in heaven" who is pleasing to Jesus, and who "will enter the kingdom of heaven."

This is a very clear statement by Jesus, and it is obvious by the exchange between Martha and Jesus concerning Lazarus that Jesus is pleased with Martha's understanding of Jesus' relationship with God.

Who performed Jesus' miracles then?

Jesus did not perform miracles - God did. This is what Martha, Jesus' follower, is saying. She understood that Jesus did not perform those miracles. This is clearly stated when she says, "God will give you whatever you ask."

She knew that Jesus asked His Beloved Supreme Being to perform them. This is critical when trying to understand who Jesus is:
"I have shown you many great miracles from the Father." (John 10:32)
Jesus was not trying to raise the dead and perform other miracles to prove that he was great. He wasn't trying to earn everyone's respect as God.

Rather, his teachings were about God: Someone Who Jesus loved, and wanted others to love: The Supreme Being.

This is why Martha said: "But I know that even now God will give you whatever you ask." She knew the Supreme Being would give Jesus whatever he asked of God because Jesus and God had a personal loving relationship. Jesus loved God and served God out of love.

And God reciprocated that love. This is called a relationship, and Jesus was trying to get each of us to also come to that point where we could exchange such a loving relationship with God.

This is why Jesus' most important instruction was:
"'Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment." (Matt. 22:37-38)
Yet it is also critical to understand why Jesus asked God to perform miracles, and why God indeed did make them happen. We can see why when we see the immediate result of these miracles:
When he [Jesus] came near the place where the road goes down the Mount of Olives, the whole crowd of disciples began joyfully to praise God in loud voices for all the miracles they had seen (Luke 19:37)
Note that these were Jesus' followers who were praising God as a result of the miracles. And they were acting in a way that was pleasing to Jesus. Why? Because they were glorifying God

Why were they praising God in 'loud voices'?

In Luke 19:37 above it says that his disciples were loudly praising God. This means to recant God's names and glories. 

Let's compare this to the recanting we often hear in political assemblies. When the crowd of a political assembly begins praising the politician they usually yell out his or her name very loudly, possibly with a slogan. The crowd might recant, for example, "Billy for congress" (fictitious name) or something, all in unison, and very loudly.

Note that Jesus' disciples also recanted Jesus' name as he walked through Mount Olives to Jerusalem. But they weren't praising Jesus alone as in a political chant. As Jesus stated in Matt. 7:21-23, he would not have been pleased with that.

Rather, the disciples were praising Jesus together with the Supreme Being. They were glorifying God and Jesus' relationship with God. That's why they shouted:
“Hosanna!” “Blessed is he who comes in the name of the Lord!” (John 12:13)