He Declares to God That He Has Been Faithful In His Testimony Towards His Fellow Believers About God's Goodness And Faithfulness, Which Is Of Course An Essential Part Of His Obedience (Psalms 40:9-10).
‘I have proclaimed glad tidings of righteousness in the great assembly,
Lo, I will not refrain my lips, O YHWH, you know.
I have not hid your righteousness within my heart,
I have declared your faithfulness and your salvation,
I have not concealed your lovingkindness,
And your truth from the great assembly.'
He declares that in the assembly and worship of the people of God he has proclaimed glad tidings of ‘righteousness'. In mind here are probably God's righteous acts in delivering His people from Philistine oppression through him (e.g. 1 Samuel 6:6-9,6:30). We must remember here that the ideas of ‘righteousness' and ‘salvation' are regularly seen in parallel. And he assures God that he will not refrain from doing so, so that all the glory might go to YHWH, something that he is assured God already knows.
Thus he can quite honestly declare that he has not hidden the truth about God's righteous activity in his heart, but has faithfully declared his faithfulness and deliverance to all. Nor has he concealed God's covenant love or His truth from them in the great assembly. In other words he assures God that he has made quite clear to the people how much they owe to God in His faithfulness and love, for their deliverance.