Throughout the history of the Church, God has allowed some of the saints to see heaven, hell, and purgatory. Here are some of their visions:

When Mary appeared to the children at Fatima, she gave them a vision of the souls in hell, which reminded them (and us!) of the importance of remaining close to God and in a state of grace. Sister Lucia, one of the children, later wrote about the terrifying sight they saw: a “sea of fire” filled with demons and the souls of those who had gone to hell.

St. Teresa of Avila had a similar vision, and later wrote:
The fact is that I don’t know how to give a sufficiently powerful description of that interior fire and that despair, coming in addition to such extreme torments and pains. I didn’t see who inflicted them on me, but, as it seemed to me, I felt myself burning and crumbling; and I repeat the worst was that interior fire and despair.  (The Collected Works of St. Teresa of Avila )

St. Faustina had a vision of purgatory which reminds us of the importance of praying for the souls in purgatory:
…I saw my Guardian Angel, who ordered me to follow him. In a moment I was in a misty place full of fire in which there was a great crowd of suffering souls. They were praying fervently, but to no avail, for themselves; only we can come to their aid. (Diary, 20)

St. Thomas Aquinas, one of the greatest writers of the Church, wrote and taught many things about God and the Church. Toward the end of his life, while celebrating Mass, he had a vision of heaven. It was so beautiful that afterward, he said that it made all of his writing—some of the greatest writing in the history of the Church—seem like straw.

There are several mentions in Scripture about visions of heaven including Saint Stephen who saw the “glory of God” (Acts 7:54-55), Saint Paul who saw such amazing things that he could not even put them into words ( 2 Corinthians 12:1-4) and Saint John who saw every living creature giving constant praise to God (Revelation 4).

Although we cannot fully understand what heaven, hell, or purgatory is like, these visions of the saints shed a little light on the mystery and give us a glimpse of what to expect. They help show us what we can look forward to in heaven, and what we want to avoid in hell.