One argument you would refute the nonbeliever with would be "Could a hired worker demand a downpayment for his salary before he started to work, or a servent before he began to serve his master all the more so a creature may not demand a pledge from his Creator before he begins to serve Him."
pg 457
If the focus in serving H'shem were to be immersed in concentration devotion and singing laments and praise to H'shem one would be commanded to say Tehillim all day. We learn that Dovid Hamelech asked H'shem to consider one who says Tehillim as though he was immersed in learning the depth of Torah, yet we are not told if H'shem agreed. This reinforces the idea that learning Torah even without the highest levels of L'Shma and concentration is obligated and desired by H'shem Yisborach.
Gate Four Chapter 2 pg 137