The first thing that we have to keep in mind is that there are many things in life that we believe in and cannot see.

There are subatomic particles that we cannot see, but we believe in it.

There are effects in nature, such as gravity, that we cannot see (although we can feel its effects), but we still believe in it.

There are laws of logic on which basis we live our lives that we cannot see, but we still know that they are real.

Therefore, believing in things that we cannot see is something we generally do, and should not be used as a basis for not believing in God. 

Secondly, by definition, God cannot be seen. God is a Spirit being (which does not have flesh and bones Luke 24:39) and therefore is not physical. Jesus Himself quotes:

John 4:24 (NKJV)

God is Spirit, and those who worship Him must worship in spirit and truth.”

Therefore, using the reasoning saying ‘you cannot believe in God because you cannot see Him’ commits what scholars call a ‘category fallacy,’ it is like saying I do not believe in the musical note ‘C’ because I cannot smell it. 

Thirdly we cannot see God during our time on earth because He now requires that men seek Him by faith. 

In the beginning, when men (Adam and Eve) did see God in the Garden of Eden, they rejected Him and broke His law.  God now requires that men believe in Him by earnestly seeking Him through faith.

Hebrews 11:6 (NKJV)

But without faith it is impossible to please Him, for he who comes to God must believe that He is, and that He is a rewarder of those who diligently seek Him.

Although we cannot see God, we can still see the full revelation and the essence of God through Jesus Christ.

John 14:9 (NKJV)

Jesus said to him, “Have I been with you so long, and yet you have not known Me, Philip? He who has seen Me has seen the Father; so how can you say, ‘Show us the Father’?

You might be thinking that it is a hard ask for anyone to believe without seeing; in fact, even Jesus’ disciple Thomas doubted that Christ had resurrected and even asked if he could physically touch the wounds of Jesus. And only then he believed.

Like Thomas, we also might have doubt, but Jesus understands that and only asks of us to have a little faith, and in turn, we would be more blessed than those who have seen: 

John 20:29 (NKJV)

Jesus said to him, “Thomas, because you have seen Me, you have believed. Blessed are those who have not seen and yet have believed.”

Conclusion

Lastly, one of the greatest hopes of the Christian church is the promise of what we call in theology the beatific vision or the vision of God as said by John in His letter;

1 John 3:2 (NKJV)

Beloved, now we are children of God; and it has not yet been revealed what we shall be, but we know that when He is revealed, we shall be like Him, for we shall see Him as He is.

John is basically saying that the very substance and essence of God, which is concealed from our eyes right now, will be seen in all of its glory, majesty, and splendor in heaven.

Until that day, the scriptures tell us consistently that no person shall see God and live; this is because God is holy, and we are not (Exod. 33:20). 

Even Moses, as righteous as he was, pleaded with God on the mountain to let him have an unveiled look at God’s glory. God only allowed him to catch a refracted glimpse of God’s back parts, but he said to Moses, “My face shall not be seen. (Exod. 33:23)” 

Ever since Adam and Eve fell and were driven from the Garden, God has been invisible to human beings, but not because God is intrinsically incapable of being seen. The problem is not with our eyes but with our hearts. 

Jesus, in the Sermon on the Mount, made the promise that someday, a certain group of people would see God.  

Matthew 5:8 (NKJV)

Blessed are the pure in heart, For they shall see God.

It’s because we’re not pure in heart that God remains invisible, and only when we’re purified will we see him.