“Strain at” in Matthew 23:24 is an old translation and is properly “strain out.” The phrase concerns filtering out a small impurity rather than the meaning suggested by the wording “strain at.”
Jesus used the expression in His rebuke of religious hypocrisy: people carefully strained out a gnat while swallowing a camel. The contrast exposes attention to a minor matter while overlooking something vastly greater.
The wording “strain at” is treated in the supplied material as a misprint or incorrect rendering in the Authorized Version, corrected to “strain out” in the Revised Version.