John saw a spectacular sight in heaven, a great multitude, along with the angels, praising God. Then one of the elders who sits on one of the thrones surrounding the throne of God, asked John whether he knew who those people were,

And I said to him, “Sir, you know.” So he said to me, “These are the ones who come out of the great tribulation, and washed their robes and made them white in the blood of the Lamb” (Revelation 7.14).

John did not know what he was seeing, but one of the elders did, explaining that the great multitude in heaven are the people who come out of tribulation, and who have washed their robes in the blood of Jesus Christ.

Therefore, we cannot go to heaven, unless we experience tribulation and have been washed in the blood of Jesus Christ.

Concerning tribulation, Paul had taught,

“We must through many tribulations enter the kingdom of God” (Acts 14.22).
Concerning coming in contact with the blood of Jesus, again Paul had taught,

“Or do you not know that as many of us as were baptized into Christ Jesus were baptized into His death? Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life” (Romans 6.3–4).

Jesus shed His blood in His death, and when we are baptized into Christ, we experience His death, coming into contact with the cleansing blood.