Fix reason firmly in her seat, and call to her tribunal every fact, every opinion. Question with boldness even the existence of a God; because, if there be one, he must more approve of the homage of reason, than that of blindfolded fear.
Thomas Jefferson

Does that preclude him from believing? I would say that and I am a Christian.

I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man.
Thomas Jefferson

How could he swear upon the altar of God if he didn’t believe?

I tremble for my country when I reflect that God is just; that his justice cannot sleep forever.
Thomas Jefferson

He must believe in a god.

Some of his statements sound very anti-Christians.

I have recently been examining all the known superstitions of the world, and do not find in our particular superstition (Christianity) one redeeming feature. They are all alike founded on fables and mythology.

In every country and every age, the priest had been hostile to Liberty.
Thomas Jefferson

I do not find in orthodox Christianity one redeeming feature.
Thomas Jefferson

But I read those and I know that hyperbole can account for some of that. And the time in which it was said can account for it. I could have said something very like these at times in my life, yet I was always a Christian.

It does me no injury for my neighbor to say there are twenty gods or no God.
Thomas Jefferson

People quote this and say, see, he doesn’t care. No. He is saying that there is no injury done to HIM if someone is a pantheist or an atheist. He is not effected. And he believed in the separation of the church (in Britain paid for by taxes) from the state. He was not saying take all religion out of government. That would have been atheistic and he knew atheism and discussed it a time or two. He meant, don’t put the church in charge of the state and don’t make the church a part of the state.

To compel a man to furnish funds for the propagation of ideas he disbelieves and abhors is sinful and tyrannical.
Thomas Jefferson

Then there are other statements Jefferson made:

The Creator has not thought proper to mark those in the forehead who are of stuff to make good generals. We are first, therefore, to seek them blindfold, and then let them learn the trade at the expense of great losses.
Thomas Jefferson

We hold these truths to be self-evident: that all men are created equal; that they are endowed by their Creator with certain unalienable rights; that among these are life, liberty, and the pursuit of happiness.
Thomas Jefferson

from Brainy Quote

These are quoted from the Jefferson Memorial:

“We hold these truths to be self-evident: that all men are created equal, that they are endowed by their Creator with certain inalienable rights, among these are life, liberty, and the pursuit of happiness, that to secure these rights governments are instituted among men. We . . . solemnly publish and declare, that these colonies are and of right ought to be free and independent states. . . And for the support of this declaration, with a firm reliance on the protection of divine providence, we mutually pledge our lives, our fortunes, and our sacred honor.”

“Well aware that the opinions and belief of men depend not on their own will, but follow involuntarily the evidence proposed to their minds; that Almighty God hath created the mind free, and manifested his supreme will that free it shall remain by making it altogether insusceptible of restraint; that all attempts to influence it by temporal punishments, or burthens, or by civil incapacitations, tend only to beget habits of hypocrisy and meanness, and are a departure from the plan of the holy author of our religion . . . .”
— “A Bill for Establishing Religious Freedom,” Section I

“But let them [members of the parliament of Great Britain] not think to exclude us from going to other markets to dispose of those commodities which they cannot use, or to supply those wants which they cannot supply. Still less let it be proposed that our properties within our own territories shall be taxed or regulated by any power on earth but our own. The God who gave us life gave us liberty at the same time; the hand of force may destroy, but cannot disjoin them.”
— “A Summary View of the Rights of British America”

“For in a warm climate, no man will labour for himself who can make another labour for him. This is so true, that of the proprietors of slaves a very small proportion indeed are ever seen to labor. And can the liberties of a nation be thought secure when we have removed their only firm basis, a conviction in the minds of the people that these liberties are the gift of God? That they are not to be violated but with his wrath? Indeed I tremble for my country when I reflect that God is just: that his justice cannot sleep for ever . . . .”
— Notes on the State of Virginia

from Monticello.org

I think he was a believer.