This question refers to several passages, including John 14:12:
I tell you the truth, anyone who has faith in Me will do what I have been doing. He will do even greater things than these, because I am going to the Father.
Jesus wasn’t saying that His disciples would be able to perform all of the supernatural acts that He did through the power of the Holy Spirit (although they did perform miracles). He was speaking of the work that He considered most important: the spread of the gospel. The Expositor’s Bible Commentary contains an interesting explanation of this verse:
He wanted to impress on the disciples that He was not disbanding them in anticipation of His departure but, rather, He was expecting them to continue His work and do even greater things than He had accomplished. Such an expectation seems impossible in the light of His character and power; yet, through the power of the Spirit whom Jesus sent after His ascension, there were more converts after the initial sermon of Peter at Pentecost than are recorded for Jesus during His entire career. The influence of the infant church covered the Roman world,whereas Jesus during His lifetime never traveled outside the boundaries of Palestine. Through the disciples He multiplied His ministry after His departure. The Book of Acts is a continuous record of deeds that followed the precedent Jesus had set. As the living Lord He continued in His church what He had himself begun. He expected that the church would become the instrument by which He could manifest His salvation to all people.
Several other passages, such as Matthew 7:7; 21:22 ; John 14:12-14 ; and 1 John 3:22-23 are often mistakenly understood to mean that God places no restrictions on what we should be able to receive in response to our prayers. But if there were no limitation on the things we could receive from God through prayer, why would Jesus say, “Blessed are those who mourn, for they will be comforted. . . .Blessed are those who are persecuted because of righteousness, for theirs is the kingdom of heaven”? ( Matthew 5:4,10 ).
In other words, if our lack of faith is all that stands in the way of our having whatever we want, we should never be mournful, persecuted,or afflicted. But that was not what Jesus promised, and His disciples did not receive everything they might have wanted. Just as Jesus had no permanent place to lay His head ( Matthew 8:20 ), the apostles suffered persecution and hardship ( 2 Corinthians 6 ), and eventually all but John were martyred.
These passages assume that we will pray in humble, childlike faith( Matthew 7:11; 17:20 ), with sincerity, out of genuine love ( Matthew 5:44 ), with good motives (Matthew 6:5 ), with perseverance ( Matthew 7:7 ), and in submission to God’s sovereign will ( Matthew 6:10 ). When we pray this way, we won’t make improper requests. Also, we will be so in tune with God that we will be satisfied when His plans prove to be different than we hoped they would be.